media: ext-ctrls-image-source.rst: document v4l2_area

The v4l2_area struct was not documented in the media documentation,
which meant that references to that struct elsewhere in the documentation
never linked to anything.

[mchehab: remove .. cssclass:: longtable]
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>