docs: mention calling register_toolchains in custom toolchain docs

Author: rickeylev

docs/toolchains.md

@@ -444,8 +444,17 @@ toolchain(
     ],
     exec_comaptible_with = ["@platforms/os:linux"]
 )
+
+# File: MODULE.bazel or WORKSPACE.bazel
+# These toolchains will considered before others
+register_toolchains("//toolchains:all")
 ```
 
+When registering custom toolchains, be aware of the the [toolchain registration
+order](https://bazel.build/extending/toolchains#toolchain-resolution). In brief,
+toolchain order is the BFS-order of the modules; see the bazel docs for a more
+detailed description.
+
 :::{note}
 The toolchain() calls should be in a separate BUILD file from everything else.
 This avoids Bazel having to perform unnecessary work when it discovers the list