Add missing docs for the ffplay -autoexit option.

Patch by Alexei Svitkine print("%s.%s@gmail.%s" % ("alexei", "svitkine", "com")).

Originally committed as revision 24038 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffplay-doc.texi

@@ -108,6 +108,8 @@ Select the desired subtitle stream number, counting from 0. The number
 refers to the list of all the input subtitle streams. If it is greater
 than the number of subtitle streams minus one, then the last one is
 selected, if it is negative the subtitle rendering is disabled.
+@item -autoexit
+Exit when video is done playing.
 @item -exitonkeydown
 Exit if any key is pressed.
 @item -exitonmousedown