fsck.gfs2(8): Manpage updates
    
    - Improve style consistency with the other manpages
    - Remove an unnecessary paragraph that gives a misleading impression of
      gfs2's device atomicity requirements (rhbz#1693000)
    - Add "See Also" section
    - "fsck.gfs" -> "fsck.gfs2"
    - Various other language tweaks
    
    Signed-off-by: Andrew Price <anprice@redhat.com>