Rollup merge of #149355 - anishamahuli:anishamahuli-issue-103557, r=Mark-Simulacrum

Document that `build.description` affects symbol mangling and crate IDs

Addresses issue #103557

Adds warning to `bootstrap.example.toml` explaining that changing build.description affects crate IDs and symbol name mangling, which makes compiled artifacts incompatible across different values.

As `@Mark-Simulacrum` points out, "if you're building with different settings, we really can't guarantee compatibility in any nice way." This documentation attempts to make that clear. This is my first contribution here, so any feedback would be greatly appreciated!

Author: Zalathar

bootstrap.example.toml

@@ -220,6 +220,11 @@
 # which is also used in places like debuginfo `DW_AT_producer`. This may be useful for
 # supplementary build information, like distro-specific package versions.
 #
+# IMPORTANT: Changing this value changes crate IDs and symbol name mangling, making
+# compiled artifacts incompatible. PGO profiles cannot be reused across different
+# descriptions, and incremental compilation caches are invalidated. Keep this value
+# consistent when reusing build artifacts.
+#
 # The Rust compiler will differentiate between versions of itself, including
 # based on this string, which means that if you wish to be compatible with
 # upstream Rust you need to set this to "". However, note that if you set this to "" but