doc: fix and clarify libx264 encoding.

tune, fastfirstpass and profile are not available through x264opts.
Since some FFmpeg options don't match the names used in libx264, add
libx264 option names as a reference when needed. And last thing, x264
don't have a manpage ATM, so prefer the x264 --fullhelp over the
unexistant manpage reference.

doc/encoders.texi

@@ -551,15 +551,12 @@ Set the encoding preset.
 
 @item tune @var{tune_name}
 Tune the encoding params.
-Deprecated in favor of @var{x264opts}
 
 @item fastfirstpass @var{bool}
 Use fast settings when encoding first pass, default value is 1.
-Deprecated in favor of @var{x264opts}.
 
 @item profile @var{profile_name}
 Set profile restrictions.
-Deprecated in favor of @var{x264opts}.
 
 @item level @var{level}
 Specify level (as defined by Annex A).
@@ -567,14 +564,14 @@ Deprecated in favor of @var{x264opts}.
 
 @item passlogfile @var{filename}
 Specify filename for 2 pass stats.
-Deprecated in favor of @var{x264opts}.
+Deprecated in favor of @var{x264opts} (see @var{stats} libx264 option).
 
 @item wpredp @var{wpred_type}
 Specify Weighted prediction for P-frames.
-Deprecated in favor of @var{x264opts}.
+Deprecated in favor of @var{x264opts} (see @var{weightp} libx264 option).
 
 @item x264opts @var{options}
-Allow to set any x264 option, see x264 manual for a list.
+Allow to set any x264 option, see x264 --fullhelp for a list.
 
 @var{options} is a list of @var{key}=@var{value} couples separated by
 ":".