media: vimc.rst: Add a proper alt attribute to vimc.dot

According to W3C, "the content of the alt attribute is: use text that
fulfills the same function as the image". While it's hard to describe
the whole content of this image, replace the actual alt to something
more useful to people with slow connection or that uses screen readers.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>