[3.12] docs: improve venv docs (GH-124540) (#124728)

[3.12] docs: improve venv docs (GH-124540) (#124728)

- 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)