[clang-tidy][docs] Rewrite bugprone check docs to Markdown [2/4] (#214419)

Tracking issue: #201242

See the [migration guide] for more information.

[migration guide]:

https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines

This rewrites part 2/4 of the remaining bugprone check documentation
from reST to MyST Markdown.

AI Usage: This was prepared with rst2myst and GPT5.6-assisted cleanup.
I manually verified that the documentation renders as expected.

Preview site:
https://broken.life/llvm-staging/bugprone-markdown-port/

GitOrigin-RevId: b563de4ea77181cd7844348f1512b32e815f9077