doc: Link to `string.capwords` from `str.title` (GH-20913)

Since `title()` mentions its own short-comings, it should also mention the library function which does not possess them.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit b786d9ec52a2c2b0b6627be7fd4a3948c61fbdea)

Co-authored-by: Eric Wieser <wieser.eric@gmail.com>