Rollup merge of #139164 - xizheyin:issue-139034, r=joboet

std: improve documentation for get_mut() methods regarding forgotten guards

Fixes #139034

This PR improves the documentation for `get_mut()` methods in `Mutex`, `RefCell`, and `RwLock` to clarify their behavior when lock guards are forgotten (e.g., via std::mem::forget).

The current documentation for these methods states that a mutable borrow "statically guarantees no locks exist", which is not entirely accurate. While a mutable borrow prevents new locks from being created, it does not clear or detect previously abandoned locks through `forget()`. This can lead to counterintuitive behavior:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=e68cefec12dcd435daf2237c16824ed3
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=81263ad652c752afd63c903113d3082c
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=311baa4edb3abf82a25c8d7bf21a4a52

r? libs

Author: matthiaskrgr