doc/utils: document channel layout specification

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@@ -579,6 +579,154 @@ The following names of colors are recognized:
 0x9ACD32
 @end table
 
+@anchor{channel layout syntax}
+@section Channel Layout
+
+A channel layout specifies the spatial disposition of the channels in
+a multi-channel audio stream. To specify a channel layout, FFmpeg
+makes use of a special syntax.
+
+Individual channels are identified by an id, as given by the table
+below:
+@table @samp
+@item FL
+front left
+@item FR
+front right
+@item FC
+front center
+@item LFE
+low frequency
+@item BL
+back left
+@item BR
+back right
+@item FLC
+front left-of-center
+@item FRC
+front right-of-center
+@item BC
+back center
+@item SL
+side left
+@item SR
+side right
+@item TC
+top center
+@item TFL
+top front left
+@item TFC
+top front center
+@item TFR
+top front right
+@item TBL
+top back left
+@item TBC
+top back center
+@item TBR
+top back right
+@item DL
+downmix left
+@item DR
+downmix right
+@item WL
+wide left
+@item WR
+wide right
+@item SDL
+surround direct left
+@item SDR
+surround direct right
+@item LFE2
+low frequency 2
+@end table
+
+Standard channel layout compositions can be specified by using the
+following identifiers:
+@table @samp
+@item mono
+FC
+@item stereo
+FL+FR
+@item 2.1
+FL+FR+LFE
+@item 3.0
+FL+FR+FC
+@item 3.0(back)
+FL+FR+BC
+@item 4.0
+FL+FR+FC+BC
+@item quad
+FL+FR+BL+BR
+@item quad(side)
+FL+FR+SL+SR
+@item 3.1
+FL+FR+FC+LFE
+@item 5.0
+FL+FR+FC+BL+BR
+@item 5.0(side)
+FL+FR+FC+SL+SR
+@item 4.1
+FL+FR+FC+LFE+BC
+@item 5.1
+FL+FR+FC+LFE+BL+BR
+@item 5.1(side)
+FL+FR+FC+LFE+SL+SR
+@item 6.0
+FL+FR+FC+BC+SL+SR
+@item 6.0(front)
+FL+FR+FLC+FRC+SL+SR
+@item hexagonal
+FL+FR+FC+BL+BR+BC
+@item 6.1
+FL+FR+FC+LFE+BC+SL+SR
+@item 6.1
+FL+FR+FC+LFE+BL+BR+BC
+@item 6.1(front)
+FL+FR+LFE+FLC+FRC+SL+SR
+@item 7.0
+FL+FR+FC+BL+BR+SL+SR
+@item 7.0(front)
+FL+FR+FC+FLC+FRC+SL+SR
+@item 7.1
+FL+FR+FC+LFE+BL+BR+SL+SR
+@item 7.1(wide)
+FL+FR+FC+LFE+BL+BR+FLC+FRC
+@item 7.1(wide-side)
+FL+FR+FC+LFE+FLC+FRC+SL+SR
+@item octagonal
+FL+FR+FC+BL+BR+BC+SL+SR
+@item downmix
+DL+DR
+@end table
+
+A custom channel layout can be specified as a sequence of terms, separated by
+'+' or '|'. Each term can be:
+@itemize
+@item
+the name of a standard channel layout (e.g. @samp{mono},
+@samp{stereo}, @samp{4.0}, @samp{quad}, @samp{5.0}, etc.)
+
+@item
+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})
+
+@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").
+
+See also the function @code{av_get_channel_layout} defined in
+@file{libavutil/channel_layout.h}.
 @c man end SYNTAX
 
 @chapter Expression Evaluation