include/bitmap.h: add new functions to documentation

I found these functions only by chance although I was looking exactly
for something like them. So, add them to the list of functions to make
them more visible.

Fixes: e837dfde ("bitmap: genericize percpu bitmap region iterators")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>