LUDOC-11 osc: document tunable parameters

LUDOC-11 osc: document tunable parameters

Add or improve the documentation for the OSC RPC parameters:
  osc.*.{checksums,checksum_type,max_dirty_mb,max_pages_per_rpc,
         max_rpcs_in_flight}

Add documetation for the llite readahead parameters:
  llite.*.{max_read_ahead_mb,max_read_ahead_per_file_mb,
           max_read_ahead_whole_mb}

Clean up the existing parameter sections to be more consistent.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ieaf09e175456b7a60d3fbd3a3ec5d020fb4d98e3
Reviewed-on: https://review.whamcloud.com/33375
Tested-by: Jenkins