[3.15] gh-154139: Document that curses is not thread-safe (GH-154173)

Whether curses is thread-safe depends on the library and how it was built.
The blocking and refresh methods release the GIL, so with a non-reentrant
curses, unsynchronized use from several threads can crash.

The reference to window.use() and screen.use() is omitted: they were added
in 3.16.

(cherry picked from commit 1ed6b78)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/curses.rst

@@ -31,6 +31,18 @@ Linux and the BSD variants of Unix.
    Whenever the documentation mentions a *character string* it can be specified
    as a Unicode string or a byte string.
 
+.. note::
+
+   Whether curses may be used from several threads
+   depends on the underlying library and how it was built.
+   In many implementations, including the default build of ncurses,
+   the screen state is shared and not thread-safe;
+   since the blocking and refresh methods
+   (such as :meth:`~window.getch` and :meth:`~window.refresh`)
+   release the :term:`GIL`,
+   unsynchronized use from several threads can then crash the interpreter.
+   Serialize the calls.
+
 .. seealso::
 
    Module :mod:`curses.ascii`