media: doc: poll: fix links to dual-ioctl sections

Links like :ref:`VIDIOC_STREAMON` expand to "ioctl VIDIOC_STREAMON,
VIDIOC_STREAMOFF". Thus our reader will think we are talking about
STREAMON _and_ STREAMOFF, but only one of the two actually applies
in some cases.

Fix by adding a link title, so the reader will read only the correct
ioctl name.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>