media: doc: pixfmt-rgb: Add title for deprecated formats

Improve readability of the documentation by adding a section title for
the deprecated formats.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/pixfmt-rgb.rst

@@ -1000,12 +1000,14 @@ devices and drivers must ignore those bits, for both
 :ref:`capture` and :ref:`output` devices.
 
 
+Deprecated RGB Formats
+======================
+
 Formats defined in :ref:`pixfmt-rgb-deprecated` are deprecated and
 must not be used by new drivers. They are documented here for reference.
 The meaning of their alpha bits ``(a)`` are ill-defined and interpreted as in
 either the corresponding ARGB or XRGB format, depending on the driver.
 
-
 .. raw:: latex
 
     \begingroup