[media] DocBook: add dmabuf as streaming I/O in VIDIOC_REQBUFS description

Commit 4b9c1cb6 ("[media] Documentation: media: description of DMABUF
importing in V4L2") documented the V4L2 dma-buf importing support but did
not update the VIDIOC_REQBUFS description, so only the Memory Mapping and
User Pointer I/O methods are mentioned.
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/DocBook/media/v4l/vidioc-reqbufs.xml

@@ -6,7 +6,7 @@
 
   <refnamediv>
     <refname>VIDIOC_REQBUFS</refname>
-    <refpurpose>Initiate Memory Mapping or User Pointer I/O</refpurpose>
+    <refpurpose>Initiate Memory Mapping, User Pointer or DMA Buffer I/O</refpurpose>
   </refnamediv>
 
   <refsynopsisdiv>