[3.13] pythongh-79149: document reentrant safety of loop.call_soon_threadsafe (pythonGH-128662) (python#128664)

pythongh-79149: document reentrant safety of `loop.call_soon_threadsafe` (pythonGH-128662)
(cherry picked from commit 4685401)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Thomas Grainger <tagrain@gmail.com>

Author: 3 people

Doc/library/asyncio-eventloop.rst

@@ -236,6 +236,9 @@ Scheduling callbacks
    another thread, this function *must* be used, since :meth:`call_soon` is not
    thread-safe.
 
+   This function is safe to be called from a reentrant context or signal handler,
+   however, it is not safe or fruitful to use the returned handle in such contexts.
+
    Raises :exc:`RuntimeError` if called on a loop that's been closed.
    This can happen on a secondary thread when the main application is
    shutting down.