Documentation: add print bitmap description

as the commit: "lib/vsprintf: implement bitmap printing through
'%*pb[l]'" add an easy way to print bitmaps. so printk-formats.txt
should reflect it.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>