documentation

Pull request #3373: feature/ZBX-20784-7.4-D

Merge in WEB/documentation from feature/ZBX-20784-7.4-D to release/7.4

* commit 'c15efbb122f2c9803a7628f81cdcc1013ec5652b':
  [ZBX-20784] updated property order
  [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