avcodec: Document that init_static_data() is not intended for time consuming operations.

Reviewed-by: Muhammad Faiz <mfcc64@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avcodec.h

@@ -3439,6 +3439,9 @@ typedef struct AVCodec {
 
     /**
      * Initialize codec static data, called from avcodec_register().
+     *
+     * This is not intended for time consuming operations as it is
+     * run for every codec regardless of that codec being used.
      */
     void (*init_static_data)(struct AVCodec *codec);