[3.13] gh-138772: Fix and improve documentation for turtle color functions (GH-139325) (GH-140048)

Use multiple signatures for clarity.
Explain different forms of bgcolor() in details.
Fix outdated docstrings.
(cherry picked from commit 525dcfe5236ee21b234cad16d2a3d5769e77e0ec)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>