doc/encoders.texi: document libvpxenc's -row-mt

added in:
734d760e lavc/libvpxenc: add -row-mt option
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Zern <jzern@google.com>

doc/encoders.texi

@@ -1679,6 +1679,8 @@ colorspaces:
 @item @samp{smpte240m} @emph{smpte240}
 @item @samp{bt2020_ncl} @emph{bt2020}
 @end table
+@item row-mt @var{boolean}
+Enable row based multi-threading.
 @end table
 
 @end table