[clang] [clang] Cleanup docs and code for legacy no_sanitize attributes (NFC). (PR #161311)

[Alexey Samsonov via cfe-commits]

vonosmas wrote:

> A release note is a good idea, just 'improved the heck outa sanitize docs' kinda thing.

Done

https://github.com/llvm/llvm-project/pull/161311