Update test exec platform documentation.

Default behavior changed at 0dd0da9.

Also space out some paragraphs more to de-densify content.

Closes #27750.

PiperOrigin-RevId: 840133157
Change-Id: Icf233c4617da7c06a5f6256f14571f703b726f24

Author: gregestren

docs/reference/test-encyclopedia.mdx

@@ -703,26 +703,35 @@ via [toolchain resolution](/extending/toolchains#toolchain-resolution), just
 like for any other action. Each test rule has an implicitly defined [
 `test` exec group](/extending/exec-groups#exec-groups-for-native-rules) that,
 unless overridden, has a mandatory toolchain requirement on
-`@bazel_tools//tools/test:default_test_toolchain_type`. Toolchains of this type
-do not carry any data in the form of providers, but can be used to influence the
-execution platform of the test action. By default, Bazel registers two such
-toolchains:
+`@bazel_tools//tools/test:default_test_toolchain_type`.
+
+Toolchains of this type do not carry any data in the form of providers, but can
+be used to influence the execution platform of the test action.
+
+Bazel registers two such toolchains, which take effect if users don't explicitly
+define their own::
 
 * If `--@bazel_tools//tools/test:incompatible_use_default_test_toolchain` is
-  disabled (the current default), the active test toolchain is
-  `@bazel_tools//tools/test:legacy_test_toolchain`. This toolchain does not
-  impose any constraints and thus test actions without manually specified exec
-  constraints are configured for the first registered execution platform. This
-  is often not the intended behavior in multi-platform builds as it can result
-  in e.g. a test binary built for Linux on a Windows machine to be executed on
-  Windows.
-* If `--@bazel_tools//tools/test:incompatible_use_default_test_toolchain` is
-  enabled, the active test toolchain is
+  enabled (**default**), the active test toolchain is
   `@bazel_tools//tools/test:default_test_toolchain`. This toolchain requires an
-  execution platform to match all the constraints of the test rule's target
-  platform. In particular, the target platform is compatible with this toolchain
+  execution platform to match all of the test rule's target platform
+  constraints.
+
+  In particular, the target platform is compatible with this toolchain
   if it is also registered as an execution platform. If no such platform is
-  found, the test rule fails with a toolchain resolution error.
+  found, the test rule fails with a toolchain resolution error
+* If `--@bazel_tools//tools/test:incompatible_use_default_test_toolchain` is
+  disabled, the active test toolchain is
+  `@bazel_tools//tools/test:legacy_test_toolchain`. This toolchain does not
+  impose any constraints and thus test actions without manually specified exec
+  constraints are configured for the first registered execution platform.
+
+  This is often not the intended behavior in multi-platform builds as it can
+  result in, for example, a test binary built for Linux on a Windows machine to
+  be executed on Windows.
+
+  As a legacy setting, expect this option to be unavailable in a future Bazel
+  release.
 
 Users can register additional toolchains for this type to influence this
 behavior and their toolchains will take precedence over the default ones.
@@ -731,8 +740,10 @@ a default toolchain for it.
 
 ## Tag conventions {:#tag-conventions}
 
-Some tags in the test rules have a special meaning. See also the
-[Bazel Build Encyclopedia on the `tags` attribute](/reference/be/common-definitions#common.tags).
+Some tags in the test rules have a special meaning.
+See also the
+[Bazel Build Encyclopedia on the `tags` attribute]
+(/reference/be/common-definitions#common.tags).
 
 <table>
   <tr>

site/en/reference/test-encyclopedia.md

@@ -706,26 +706,35 @@ via [toolchain resolution](/extending/toolchains#toolchain-resolution), just
 like for any other action. Each test rule has an implicitly defined [
 `test` exec group](/extending/exec-groups#exec-groups-for-native-rules) that,
 unless overridden, has a mandatory toolchain requirement on
-`@bazel_tools//tools/test:default_test_toolchain_type`. Toolchains of this type
-do not carry any data in the form of providers, but can be used to influence the
-execution platform of the test action. By default, Bazel registers two such
-toolchains:
+`@bazel_tools//tools/test:default_test_toolchain_type`.
+
+Toolchains of this type don't carry any data in the form of providers, but can
+be used to influence the execution platform of the test action.
+
+Bazel registers two such toolchains, which take effect if users don't explicitly
+define their own::
 
 * If `--@bazel_tools//tools/test:incompatible_use_default_test_toolchain` is
-  disabled (the current default), the active test toolchain is
-  `@bazel_tools//tools/test:legacy_test_toolchain`. This toolchain does not
-  impose any constraints and thus test actions without manually specified exec
-  constraints are configured for the first registered execution platform. This
-  is often not the intended behavior in multi-platform builds as it can result
-  in e.g. a test binary built for Linux on a Windows machine to be executed on
-  Windows.
-* If `--@bazel_tools//tools/test:incompatible_use_default_test_toolchain` is
-  enabled, the active test toolchain is
+  enabled (**default**), the active test toolchain is
   `@bazel_tools//tools/test:default_test_toolchain`. This toolchain requires an
-  execution platform to match all the constraints of the test rule's target
-  platform. In particular, the target platform is compatible with this toolchain
+  execution platform to match all of the test rule's target platform
+  constraints.
+
+  In particular, the target platform is compatible with this toolchain
   if it is also registered as an execution platform. If no such platform is
-  found, the test rule fails with a toolchain resolution error.
+  found, the test rule fails with a toolchain resolution error
+* If `--@bazel_tools//tools/test:incompatible_use_default_test_toolchain` is
+  disabled, the active test toolchain is
+  `@bazel_tools//tools/test:legacy_test_toolchain`. This toolchain does not
+  impose any constraints and thus test actions without manually specified exec
+  constraints are configured for the first registered execution platform.
+
+  This is often not the intended behavior in multi-platform builds as it can
+  result in, for example, a test binary built for Linux on a Windows machine to
+  be executed on Windows.
+
+  As a legacy setting, expect this option to be unavailable in a future Bazel
+  release.
 
 Users can register additional toolchains for this type to influence this
 behavior and their toolchains will take precedence over the default ones.
@@ -734,8 +743,10 @@ a default toolchain for it.
 
 ## Tag conventions {:#tag-conventions}
 
-Some tags in the test rules have a special meaning. See also the
-[Bazel Build Encyclopedia on the `tags` attribute](/reference/be/common-definitions#common.tags).
+Some tags in the test rules have a special meaning.
+See also the
+[Bazel Build Encyclopedia on the `tags` attribute]
+(/reference/be/common-definitions#common.tags).
 
 <table>
   <tr>