doc/formats.texi: document max_interlave_delta muxing option

The option was added in commit d9ae1031f5
but it was never documented. The text is based on the one written by the
commit author for the corresponding AVFormatContext.max_interleave_delta
field.

doc/formats.texi

@@ -127,6 +127,25 @@ Consider all spec non compliancies as errors.
 Consider things that a sane encoder should not do as an error.
 @end table
 
+@item max_interleave_delta @var{integer} (@emph{output})
+Set maximum buffering duration for interleaving. The duration is
+expressed in microseconds, and defaults to 1000000 (1 second).
+
+To ensure all the streams are interleaved correctly, libavformat will
+wait until it has at least one packet for each stream before actually
+writing any packets to the output file. When some streams are
+"sparse" (i.e. there are large gaps between successive packets), this
+can result in excessive buffering.
+
+This field specifies the maximum difference between the timestamps of the
+first and the last packet in the muxing queue, above which libavformat
+will output a packet regardless of whether it has queued a packet for all
+the streams.
+
+If set to 0, libavformat will continue buffering packets until it has
+a packet for each stream, regardless of the maximum timestamp
+difference between the buffered packets.
+
 @item use_wallclock_as_timestamps @var{integer} (@emph{input})
 Use wallclock as timestamps.