bitmap: extend comment to bitmap_print_bitmask/list_to_buf

Extend comment to new function to warn potential users about caveats.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Link: https://lore.kernel.org/r/20210806110251.560-6-song.bao.hua@hisilicon.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>