gh-153862: Suggest instr() for non-encodable characters read by inch()

A character that does not fit in a single byte has a character byte of 0
in the value returned by inch(); document reading such characters with
instr().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/curses.rst

@@ -1081,6 +1081,9 @@ Window objects
    and the color pair with :func:`pair_number`.
    The character byte is the locale-encoded byte of the cell's character,
    consistent with :meth:`instr`.
+   On a wide-character build, a character that does not fit in a single byte
+   in the current locale has a character byte of ``0``;
+   use :meth:`instr` to read such characters.
 
 
 .. method:: window.insch(ch[, attr])