Fixes some mistakes and misleadings in the quote function docstring:
- reserved chars are never actually used by quote code, unreserved chars are
- reserved chars were wrong and incomplete
- mentioned that use-case is not minimal quoting wrt. RFC, but cautious quoting
(cherry picked from commit 750d74f)

Co-authored-by: Jörn Hees <joernhees@users.noreply.github.com>