block: document usage of bio iterator helpers

   size limitations and the limitations of the underlying devices. Thus

   there's no need to define ->merge_bvec_fn() callbacks for individual block

   drivers.

Usage of helpers:

=================

* The following helpers whose names have the suffix of "_all" can only be used

on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers

shouldn't use them because the bio may have been split before it reached the

driver.

	bio_for_each_segment_all()

	bio_first_bvec_all()

	bio_first_page_all()

	bio_last_bvec_all()

* The following helpers iterate over single-page segment. The passed 'struct

bio_vec' will contain a single-page IO vector during the iteration

	bio_for_each_segment()

	bio_for_each_segment_all()

* The following helpers iterate over multi-page bvec. The passed 'struct

bio_vec' will contain a multi-page IO vector during the iteration

	bio_for_each_bvec()

	rq_for_each_bvec()