docs: atomic_bitops.txt: add a title for this document

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Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/atomic_bitops.txt

-
-On atomic bitops.
-
+=============
+Atomic bitops
+=============
 
 While our bitmap_{}() functions are non-atomic, we have a number of operations
 operating on single bits in a bitmap that are atomic.