docs: clarify stdlib monkeypatch guidance

[MukundaKatta]

Document safer patterns for monkeypatching standard library functions:

• prefer patching the reference used by the code under test
• prefer explicit dependencies for code users control
• use MonkeyPatch.context() to keep unavoidable stdlib patches narrowly scoped

Closes #11022

Verification:

• sphinx-build -W --keep-going -b html doc/en doc/en/_build/html
• git diff --check