Document that build.description affects symbol mangling and crate IDs

[anishamahuli]

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!

[rustbot]

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit 4af0b97 has been approved by Mark-Simulacrum

It is now in the queue for this repository.