doc/demuxers: note defaults for apng options

doc/demuxers.texi

@@ -37,12 +37,15 @@ between the last fcTL and IEND chunks.
 
 @table @option
 @item -ignore_loop @var{bool}
-Ignore the loop variable in the file if set.
+Ignore the loop variable in the file if set. Default is enabled.
+
 @item -max_fps @var{int}
-Maximum framerate in frames per second (0 for no limit).
+Maximum framerate in frames per second. Default of 0 imposes no limit.
+
 @item -default_fps @var{int}
 Default framerate in frames per second when none is specified in the file
-(0 meaning as fast as possible).
+(0 meaning as fast as possible). Default is 15.
+
 @end table
 
 @section asf