Updates to mpr driver man page.

• Add 3216 and 3224 support.
• Add SSU, chain_alloc_fail, and spinup_wait_time information.
• Clear up some sentences.
• Correct some typos.

Reviewed by: ken, scottl, ambrisko, asomers, wblock
Approved by: ken, scottl, ambrisko
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6104