doc: document cutoff option to ac3 and adjust the option's global documentation

cutoff is implemented as an option global to lavc, but supported only
by a few encoders. This fact is now reflected in its documentation. ac3's
support of this option is added for completeness.
Signed-off-by: NMoritz Barsnick <barsnick@gmx.net>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

doc/codecs.texi

@@ -138,7 +138,8 @@ Set audio sampling rate (in Hz).
 Set number of audio channels.
 
 @item cutoff @var{integer} (@emph{encoding,audio})
-Set cutoff bandwidth.
+Set cutoff bandwidth. (Supported only by selected encoders, see
+their respective documentation sections.)
 
 @item frame_size @var{integer} (@emph{encoding,audio})
 Set audio frame size.

doc/encoders.texi

@@ -488,6 +488,10 @@ is an optional AC-3 feature that increases quality by selectively encoding
 the left/right channels as mid/side. This option is enabled by default, and it
 is highly recommended that it be left as enabled except for testing purposes.
 
+@item cutoff @var{frequency}
+Set lowpass cutoff frequency. If unspecified, the encoder selects a default
+determined by various other encoding parameters.
+
 @end table
 
 @subsection Floating-Point-Only AC-3 Encoding Options