gh-77171: Document that wave only supports simple PCM files (GH-97510)

(cherry picked from commit dc9065f8c2ddc483d387d977e0818d131fa67420)

Co-authored-by: Steve Dower <steve.dower@python.org>

Doc/library/wave.rst

diff --git a/Doc/library/wave.rst b/Doc/library/wave.rst
index f63e0d3..d50aabd 100644
--- a/Doc/library/wave.rst
+++ b/Doc/library/wave.rst
@@ -12,7 +12,8 @@
 --------------
 
 The :mod:`wave` module provides a convenient interface to the WAV sound format.
-It does not support compression/decompression, but it does support mono/stereo.
+Only files using ``WAVE_FORMAT_PCM`` are supported. Note that this does not
+include files using ``WAVE_FORMAT_EXTENSIBLE`` even if the subformat is PCM.
 
 The :mod:`wave` module defines the following function and exception: