gh-79964: Clarify the imaplib fetch example in the docs (GH-152780)

A FETCH response may contain additional or unsolicited data, so the
message is not always data[0][1].

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/imaplib.rst

@@ -730,3 +730,10 @@ retrieves and prints all messages::
    M.close()
    M.logout()
 
+.. note::
+
+   A ``FETCH`` response may contain additional or unsolicited data
+   (see :rfc:`3501`, section 7.4.2),
+   so production code should inspect the whole response
+   rather than rely on ``data[0][1]``.
+