document stability issues around libraries

Author: jyn514

Diff for: src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md

@@ -38,6 +38,13 @@ Like any other unstable feature, we reserve the right to change or remove this f
 Although we do not take technical measures to prevent it from being used, we strongly discourage using this feature.
 If at all possible, please contribute to stabilizing the features you care about instead of bypassing the Rust project's stability policy.
 
+For library crates, we especially discourage the use of this feature.
+The crates depending on you do not know that you use this feature, have little recourse if it breaks, and can be used in contexts that are hard to predict.
+
+For libraries that do use this feature, please document the versions you support (including a *maximum* as well as minimum version), and a mechanism to disable it.
+If you do not have a mechanism to disable the use of `RUSTC_BOOTSTRAP`, consider removing its use altogether, such that people can only use your library if they are already using a nightly toolchain.
+This leaves the choice of whether to opt-out of Rust's stability promise up to the end user building their code.
+
 ## History
 
 - [Allowed without a hash](https://github.com/rust-lang/rust/pull/37265) ([discussion](https://github.com/rust-lang/rust/issues/36548))