doc/ffserver: document "Launch" feed option

doc/ffserver.texi

@@ -335,7 +335,21 @@ Set maximum size of the feed. 0 means unlimited.
 @item ReadOnlyFile @var{filename}
 Mark the file as readonly and it will not be deleted or updated.
 
-@item Launch
+@item Launch @var{args}
+Launch an @command{ffmpeg} command when creating @command{ffserver}.
+
+@var{args} must be a sequence of arguments to be provided to an
+@command{ffmpeg} instance. The first provided argument is ignored, and
+it is replaced by a path with the same dirname of the @command{ffserver}
+instance, followed by the remaining argument and terminated with a
+path corresponding to the feed.
+
+In case you need a more complex @command{ffmpeg} configuration,
+e.g. if you need to generate multiple FFM feeds with a single
+@command{ffmpeg} instance, you should launch @command{ffmpeg} by hand.
+
+This option is ignored in case the commandline option @option{-n} is
+specified.
 
 @item ACL
 Specify the list of IP address which are allowed or denied to write