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: 1825788 ("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>

Documentation/userspace-api/media/frontend.h.rst.exceptions

@@ -142,6 +142,10 @@ ignore symbol FEC_26_45
 ignore symbol FEC_28_45
 ignore symbol FEC_32_45
 ignore symbol FEC_77_90
+ignore symbol FEC_11_45
+ignore symbol FEC_4_15
+ignore symbol FEC_14_45
+ignore symbol FEC_7_15
 
 ignore symbol TRANSMISSION_MODE_AUTO
 ignore symbol TRANSMISSION_MODE_1K