LUDOC-251 hsm: Fix typos and wording in HSM section

LUDOC-251 hsm: Fix typos and wording in HSM section

Fixed the HSM event names in section 22.7.  They now match the enum
hsm_event in the Lustre source, and their names now match their
descriptions.

Also fixed minor English usage mistakes in an effort to make the
HSM chapter more readable.

Signed-off-by: Ryan Haasken <haasken@cray.com>
Change-Id: Ib331fce9a7789a8ee51f8063c5d0ddf9dd696887
Reviewed-on: http://review.whamcloud.com/11223
Tested-by: Jenkins
Reviewed-by: Richard Henwood <richard.henwood@intel.com>