docs: clearing-warn-once.txt: add a title for this document

This document misses a title. Add it, in order to follow
the documentation standard.
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/clearing-warn-once.txt

+Clearing WARN_ONCE
+------------------
 
 WARN_ONCE / WARN_ON_ONCE only print a warning once.