[media] doc_rst: media: New SDR formats PC16, PC18 & PC20

This patch adds documentation for the three new SDR formats

V4L2_SDR_FMT_PCU16BE
V4L2_SDR_FMT_PCU18BE
V4L2_SDR_FMT_PCU20BE

Signed-off-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>