[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 d831a69)
(cherry picked from commit 0901331)

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

Author: 3 people

Doc/library/curses.rst

@@ -955,6 +955,7 @@ Window objects
 .. method:: window.getbkgd()
 
    Return the given window's current background character/attribute pair.
+   Its components can be extracted like those of :meth:`inch`.
 
 
 .. method:: window.getch([y, x])
@@ -1042,8 +1043,10 @@ Window objects
 
 .. method:: window.inch([y, x])
 
-   Return the character at the given position in the window. The bottom 8 bits are
-   the character proper, and upper bits are the attributes.
+   Return the character at the given position in the window.
+   The bottom 8 bits are the character proper and the upper bits are the attributes;
+   extract them with the :data:`A_CHARTEXT` and :data:`A_ATTRIBUTES` bit-masks,
+   and the color pair with :func:`pair_number`.
 
 
 .. method:: window.insch(ch[, attr])