doc/decoders: adding documentation for lavc/dvdsubdec.c option "forced_subs_only"

Signed-off-by: Nicholas Robbins <nickrobbins@yahoo.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/decoders.texi

@@ -190,6 +190,11 @@ The format for this option is a string containing 16 24-bits hexadecimal
 numbers (without 0x prefix) separated by comas, for example @code{0d00ee,
 ee450d, 101010, eaeaea, 0ce60b, ec14ed, ebff0b, 0d617a, 7b7b7b, d1d1d1,
 7b2a0e, 0d950c, 0f007b, cf0dec, cfa80c, 7c127b}.
+
+@item forced_subs_only
+Only decode subtitle entries marked as forced. Some titles have forced
+and non-forced subtitles in the same track. Setting this flag to @code{1}
+will only keep the forced subtitles. Default value is @code{0}.
 @end table
 
 @section libzvbi-teletext