[3.13] gh-87881: Document the result of curses inch() and getbkgd() (GH-152488) ...

[3.13] gh-87881: Document the result of curses inch() and getbkgd() (GH-152488) (GH-152511) (GH-152515)

Explain the character/attribute bit layout and how to extract the parts
(A_CHARTEXT and A_ATTRIBUTES bit-masks, pair_number() for the color pair).
(cherry picked from commit d831a690766e92b3be5e40134f64407bd041bcbf)
(cherry picked from commit 09013312bbdf9ff6583c95ba6bc2698e61c66be6)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>