[3.13] gh-137986: Fix and improve the csv functions docstrings (GH-137987) (GH-138108)

The csv.register_dialect() docstring no longer imply that it returns a
dialect.
All functions have now signatures.
(cherry picked from commit aa1dbd4dde32951de2e7438b56d6761001604ee2)

Co-authored-by: maurycy <5383+maurycy@users.noreply.github.com>