doc/muxers: add AVI muxer documentation

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Tobias Rapp <t.rapp@noa-archive.com>

doc/muxers.texi

@@ -57,6 +57,39 @@ fragmentation or muxer overhead depending on your source. Default value is
 
 @end table
 
+@anchor{avi}
+@section avi
+
+Audio Video Interleaved muxer.
+
+@subsection Options
+
+It accepts the following options:
+
+@table @option
+@item reserve_index_space
+Reserve the specified amount of bytes for the OpenDML master index of each
+stream within the file header. By default additional master indexes are
+embedded within the data packets if there is no space left in the first master
+index and are linked together as a chain of indexes. This index structure can
+cause problems for some use cases, e.g. third-party software strictly relying
+on the OpenDML index specification or when file seeking is slow. Reserving
+enough index space in the file header avoids these problems.
+
+The required index space depends on the output file size and should be about 16
+bytes per gigabyte. When this option is omitted or set to zero the necessary
+index space is guessed.
+
+@item write_channel_mask
+Write the channel layout mask into the audio stream header.
+
+This option is enabled by default. Disabling the channel mask can be useful in
+specific scenarios, e.g. when merging multiple audio streams into one for
+compatibility with software that only supports a single audio stream in AVI
+(see @ref{amerge,,the "amerge" section in the ffmpeg-filters manual,ffmpeg-filters}).
+
+@end table
+
 @anchor{chromaprint}
 @section chromaprint