Make clear in the doxy that this is a *video* filter.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20298 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavfilter/vf_null.c

@@ -18,7 +18,7 @@
 
 /**
  * @file libavfilter/vf_null.c
- * null filter
+ * null video filter
  */
 
 #include "avfilter.h"