doc: Added background discussion to clock settings.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added additional background to address additional states.

Covers 1-node cluster issues, fewer OSDs than replicas, and
active + clean + inconsistent states in greater detail.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added a tip to install NTP when running multiple monitors.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>