doc/examples/decode_video: add input file format information for usage

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

doc/examples/decode_video.c

@@ -95,7 +95,8 @@ int main(int argc, char **argv)
     AVPacket *pkt;
 
     if (argc <= 2) {
-        fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
+        fprintf(stderr, "Usage: %s <input file> <output file>\n"
+                "And check your input file is encoded by mpeg1video please.\n", argv[0]);
         exit(0);
     }
     filename    = argv[1];