docs: mm/gup: minor documentation update

pin_user_pages*() APIs are clearly distinct from the get_user_pages*() APIs, so

that's a natural dividing line, and a good point to make separate wrapper calls.

In other words, use pin_user_pages*() for DMA-pinned pages, and

get_user_pages*() for other cases. There are four cases described later on in

get_user_pages*() for other cases. There are five cases described later on in

this document, to further clarify that concept.

FOLL_PIN and FOLL_GET are mutually exclusive for a given gup call. However,