[media] doc-rst: add dmabuf as streaming I/O in VIDIOC_REQBUFS description

Commit 707e65831d3b("[media] DocBook: add dmabuf as streaming I/O
in VIDIOC_REQBUFS description") added DMABUF to reqbufs description,
but, as we're migrating to ReST markup, we need to keep it in sync
with the change.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/vidioc-reqbufs.rst

@@ -9,7 +9,7 @@ ioctl VIDIOC_REQBUFS
 Name
 ====
 
-VIDIOC_REQBUFS - Initiate Memory Mapping or User Pointer I/O
+VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
 
 
 Synopsis