[3.12] gh-123494: Improve documentation for ``webbrowser`` return types (GH-123495) (#123549)

gh-123494: Improve documentation for ``webbrowser`` return types (GH-123495)

Document the return value for ``webbrowser.open*()``.
(cherry picked from commit 0b6acfee04b30e7993314723c614625ddd90ae6e)

Co-authored-by: Aarni Koskela <akx@iki.fi>