Document that `build.description` affects symbol mangling and crate IDs Addresses issue rust-lang/rust#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!