media: docs: selection: improve formatting

Split section "Comparison with old cropping API" in paragraphs for
easier reading and improve visible links text.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>