docs: uapi: media: ignore new DVB-S2X FEC values

Some new FEC values was added to support DVB-S2X. They're properly
documented, but its addition adds some extra warnings to htmldocs
build. So, add them to the ignore list.

Fixes: 1825788e ("media: dvb: add missing DVB-S2X FEC parameter values")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>