avio: Document that write_packet() is not allowed to change its input

Found-by: Reimar
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avio.h

@@ -171,6 +171,7 @@ int avio_check(const char *url, int flags);
  * @param opaque An opaque pointer to user-specific data.
  * @param read_packet  A function for refilling the buffer, may be NULL.
  * @param write_packet A function for writing the buffer contents, may be NULL.
+ *        The function may not change the input buffers content.
  * @param seek A function for seeking to specified byte position, may be NULL.
  *
  * @return Allocated AVIOContext or NULL on failure.