avcodec/get_bits: Document the return code of get_vlc2()

Found-by: kierank
Reviewed-by: Kieran Kunhya <kieran618@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4a94ff4ccd)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/get_bits.h

@@ -550,6 +550,7 @@ static inline const uint8_t *align_get_bits(GetBitContext *s)
  * @param max_depth is the number of times bits bits must be read to completely
  *                  read the longest vlc code
  *                  = (max_vlc_length + bits - 1) / bits
+ * @returns the code parsed or -1 if no vlc matches
  */
 static av_always_inline int get_vlc2(GetBitContext *s, VLC_TYPE (*table)[2],
                                      int bits, int max_depth)