avcodec/bitstream: Document the values supported for *_size in ff_init_vlc_sparse()

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/bitstream.c

@@ -266,7 +266,7 @@ static int build_table(VLC *vlc, int table_nb_bits, int nb_codes,
    'bits' or 'codes' tables.
 
    'xxx_size' : gives the number of bytes of each entry of the 'bits'
-   or 'codes' tables.
+   or 'codes' tables. Currently 1,2 and 4 are supported.
 
    'wrap' and 'size' make it possible to use any memory configuration and types
    (byte/word/long) to store the 'bits', 'codes', and 'symbols' tables.