HBASE-12544 restore ops_mgt section of ref guide.

* checked out file version as of d7f51e^
* redid cherry-pick of changes to ops_mgt.xml from master commit 804444
* fixup with cherry pick of 71ed3b to replace mdash entity with unicode.

Original Author: stack <stack@apache.org>

Signed-off-by: stack <stack@apache.org>