documentation

Pull request #3356: feature/ZBX-20784-7.5-D

Merge in WEB/documentation from feature/ZBX-20784-7.5-D to release/8.0

* commit 'a3db97674c22010263df8c740be94561049fb9af':
  [ZBX-20784; ZBXNEXT-1527] removed duplicate property
  [ZBX-20784] updated links from "evaltype" and "operator" in service.get
  [ZBX-20784] clarified meaning of each SLI reporting period
  [ZBX-20784] fixed "periods" data type; updated "period_from" and "period_to" parameter descriptions; updated existing and added new sla.getsli example
  [ZBX-20784] limited "selectParents" output in service.get
  [ZBX-20784] added links to clarify "selectChildren" and "selectParents", and updated related example
  [ZBX-20784] added links to clarify "evaltype" and "operator"; fixed possible value formatting