[3.12] gh-126137: improve docs for loop.add_reader and loop.add_writer (GH-128666) (#128668)

gh-126137: improve docs for `loop.add_reader` and `loop.add_writer` (GH-128666)
(cherry picked from commit b2adf55)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Author: miss-islington

Doc/library/asyncio-eventloop.rst

@@ -944,6 +944,9 @@ Watching file descriptors
    invoke *callback* with the specified arguments once *fd* is available for
    reading.
 
+   Any preexisting callback registered for *fd* is cancelled and replaced by
+   *callback*.
+
 .. method:: loop.remove_reader(fd)
 
    Stop monitoring the *fd* file descriptor for read availability. Returns
@@ -955,6 +958,9 @@ Watching file descriptors
    invoke *callback* with the specified arguments once *fd* is available for
    writing.
 
+   Any preexisting callback registered for *fd* is cancelled and replaced by
+   *callback*.
+
    Use :func:`functools.partial` :ref:`to pass keyword arguments
    <asyncio-pass-keywords>` to *callback*.