[3.13] docs: improve venv docs (GH-124540) (#124727)

docs: improve venv docs (GH-124540)

- Move "versionchanged" notes that apply to the whole class to the
  end of the class docs
- Remove or move notes next to the method list that apply to individual
  methods.
- Mark up parameters using the appropriate syntax
- Do not capitalize "boolean"
- Shorten some text
(cherry picked from commit c976d789a98047ae7ddec6d13c9ea7086d9fa3f9)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>