avutil/channel_layout: fix remains of old syntax in docs and comments

Signed-off-by: Marton Balint <cus@passwd.hu>

doc/utils.texi

@@ -719,19 +719,19 @@ the name of a standard channel layout (e.g. @samp{mono},
 the name of a single channel (e.g. @samp{FL}, @samp{FR}, @samp{FC}, @samp{LFE}, etc.)
 
 @item
-a number of channels, in decimal, optionally followed by 'c', yielding
-the default channel layout for that number of channels (see the
-function @code{av_get_default_channel_layout})
+a number of channels, in decimal, followed by 'c', yielding the default channel
+layout for that number of channels (see the function
+@code{av_get_default_channel_layout}). Note that not all channel counts have a
+default layout.
 
 @item
 a channel layout mask, in hexadecimal starting with "0x" (see the
 @code{AV_CH_*} macros in @file{libavutil/channel_layout.h}.
 @end itemize
 
-Starting from libavutil version 53 the trailing character "c" to
-specify a number of channels will be required, while a channel layout
-mask could also be specified as a decimal number (if and only if not
-followed by "c").
+Before libavutil version 53 the trailing character "c" to specify a number of
+channels was optional, but now it is required, while a channel layout mask can
+also be specified as a decimal number (if and only if not followed by "c").
 
 See also the function @code{av_get_channel_layout} defined in
 @file{libavutil/channel_layout.h}.

libavutil/channel_layout.h

@@ -131,17 +131,12 @@ enum AVMatrixEncoding {
  *   5.0(side), 5.1, 5.1(side), 7.1, 7.1(wide), downmix);
  * - the name of a single channel (FL, FR, FC, LFE, BL, BR, FLC, FRC, BC,
  *   SL, SR, TC, TFL, TFC, TFR, TBL, TBC, TBR, DL, DR);
- * - a number of channels, in decimal, optionally followed by 'c', yielding
+ * - a number of channels, in decimal, followed by 'c', yielding
  *   the default channel layout for that number of channels (@see
  *   av_get_default_channel_layout);
  * - a channel layout mask, in hexadecimal starting with "0x" (see the
  *   AV_CH_* macros).
  *
- * @warning Starting from the next major bump the trailing character
- * 'c' to specify a number of channels will be required, while a
- * channel layout mask could also be specified as a decimal number
- * (if and only if not followed by "c").
- *
  * Example: "stereo+FC" = "2c+FC" = "2c+1c" = "0x7"
  */
 uint64_t av_get_channel_layout(const char *name);