mm/hmm: documentation editorial update to HMM documentation

Update the documentation for HMM to fix minor typos and phrasing to be a
bit more readable.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ralph Campbell <[email protected]>
Signed-off-by: Jérôme Glisse <[email protected]>
Cc: Stephen  Bates <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: Evgeny Baskakov <[email protected]>
Cc: Mark Hairgrove <[email protected]>
Cc: John Hubbard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Ralph Campbell