Add a comment explaining Sphinx 6.2 version

Author: vstinner

Doc/requirements.txt

@@ -6,6 +6,7 @@
 # Sphinx version is pinned so that new versions that introduce new warnings
 # won't suddenly cause build failures. Updating the version is fine as long
 # as no warnings are raised by doing so.
+# PR #104777: Sphinx 6.2 no longer uses imghdr, removed in Python 3.13.
 sphinx==6.2.0
 
 blurb