Add caution note about different bit size for METADATA_BLOCK_HEADER (24) and METADATA_BLOCK_PICTURE (32) length

[wader]

There are software out there that ends up writing a truncated length in METADATA_BLOCK_PICTURE. ffmpeg used to write truncated size (now it fails instead) and now also has a workaround for decoding truncated files (https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/flac_picture.c#L129).