mm: add pagemap.h to the fine documentation

The documentation currently does not include the deathless prose written
to describe functions in pagemap.h because it's not included in any rst
file.  Fix up the mismatches between parameter names and the documentation
and add the file to mm-api.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: http://lkml.kernel.org/r/20200221220045.24989-1-willy@infradead.org


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>