qapi/qapi-schema: Update introduction for example notation

The introduction explains example notation.  The series merged in
merge commit e6485190f77e (in 9.1) improved how they look in generated
docs, but neglected to update the introduction accordingly.  Do that
now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250404121413.1743790-9-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qapi/qapi-schema.json

@@ -26,10 +26,10 @@
 #
 # Example:
 #
-# ::
+# .. qmp-example::
 #
-#   -> data issued by the Client
-#   <- Server data response
+#   -> ... text sent by client (commands) ...
+#   <- ... text sent by server (command responses and events) ...
 #
 # Please refer to the
 # :doc:`QEMU Machine Protocol Specification </interop/qmp-spec>`