gh-90923: Improve sqlite3.Connection.execute* docs (GH-91643)

- Drop 'nonstandard'; it does not add any value
- Try to be more concise
- Make return value a little more explicit
(cherry picked from commit 017f07a229a337e9c17bed8cd1879e0177a8d89d)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>