doc/ffmpeg: document trailing "?" in map option

This feature was added in 2375a85c.
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/ffmpeg.texi

@@ -871,7 +871,7 @@ Set the size of the canvas used to render subtitles.
 @section Advanced options
 
 @table @option
-@item -map [-]@var{input_file_id}[:@var{stream_specifier}][,@var{sync_file_id}[:@var{stream_specifier}]] | @var{[linklabel]} (@emph{output})
+@item -map [-]@var{input_file_id}[:@var{stream_specifier}][?][,@var{sync_file_id}[:@var{stream_specifier}]] | @var{[linklabel]} (@emph{output})
 
 Designate one or more input streams as a source for the output file. Each input
 stream is identified by the input file index @var{input_file_id} and
@@ -887,6 +887,11 @@ the source for output stream 1, etc.
 A @code{-} character before the stream identifier creates a "negative" mapping.
 It disables matching streams from already created mappings.
 
+A trailing @code{?} after the stream index will allow the map to be
+optional: if the map matches no streams the map will be ignored instead
+of failing. Note the map will still fail if an invalid input file index
+is used; such as if the map refers to a non-existant input.
+
 An alternative @var{[linklabel]} form will map outputs from complex filter
 graphs (see the @option{-filter_complex} option) to the output file.
 @var{linklabel} must correspond to a defined output link label in the graph.
@@ -924,6 +929,13 @@ To map all the streams except the second audio, use negative mappings
 ffmpeg -i INPUT -map 0 -map -0:a:1 OUTPUT
 @end example
 
+To map the video and audio streams from the first input, and using the
+trailing @code{?}, ignore the audio mapping if no audio streams exist in
+the first input:
+@example
+ffmpeg -i INPUT -map 0:v -map 0:a? OUTPUT
+@end example
+
 To pick the English audio stream:
 @example
 ffmpeg -i INPUT -map 0:m:language:eng OUTPUT