doc: add decoders.texi file

doc/decoders.texi

+@chapter Decoders
+@c man begin DECODERS
+
+Decoders are configured elements in FFmpeg which allow the decoding of
+multimedia streams.
+
+When you configure your FFmpeg build, all the supported native decoders
+are enabled by default. Decoders requiring an external library must be enabled
+manually via the corresponding @code{--enable-lib} option. You can list all
+available decoders using the configure option @code{--list-decoders}.
+
+You can disable all the decoders with the configure option
+@code{--disable-decoders} and selectively enable / disable single decoders
+with the options @code{--enable-decoder=@var{DECODER}} /
+@code{--disable-decoder=@var{DECODER}}.
+
+The option @code{-codecs} of the ff* tools will display the list of
+enabled decoders.
+
+A description of some of the currently available decoders follows.
+
+@c man end DECODERS

doc/ffmpeg.texi

@@ -1052,6 +1052,7 @@ file to which you want to add them.
 @c man end EXAMPLES
 
 @include eval.texi
+@include decoders.texi
 @include encoders.texi
 @include demuxers.texi
 @include muxers.texi

doc/ffplay.texi

@@ -174,6 +174,7 @@ Seek to percentage in file corresponding to fraction of width.
 @c man end
 
 @include eval.texi
+@include decoders.texi
 @include demuxers.texi
 @include muxers.texi
 @include indevs.texi

doc/ffprobe.texi

@@ -111,6 +111,7 @@ with name "STREAM".
 @end table
 @c man end
 
+@include decoders.texi
 @include demuxers.texi
 @include protocols.texi
 @include indevs.texi