[3.13] gh-126137: improve docs for `loop.add_reader` and `loop.add_writer` (GH-128666) (#128667)

gh-126137: improve docs for `loop.add_reader` and `loop.add_writer` (GH-128666)
(cherry picked from commit b2adf556747d080f04b53ba4063b627c2dbe41d1)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>