documentation

Pull request #2371: ZBX-21821 added a note for recommended expression for the widget parameters.

Merge in WEB/documentation from feature/ZBX-21821-7.2-D to release/7.2

* commit '55a4bc386fff101a920ea865c1669e75635844e4':
  ZBX-21821 added link, note simplification.
  ZBX-21821 changed the location of the note.
  ZBX-21821 added a note for recommended expression for the widget parameters.