bitops: Add a comment explaining the double underscore macros

Linus Walleij pointed out that a new comer might be confused about the
difference between set_bit() and __set_bit().  Add a comment explaining
the difference.

Link: https://lore.kernel.org/all/CACRpkdZFPG_YLici-BmYfk9HZ36f4WavCN3JNotkk8cPgCODCg@mail.gmail.com/


Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>