documentation

Pull request #2372: ZBX-21821 added a note for recommended expression for the widget parameters.

Merge in WEB/documentation from feature/ZBX-21821-7.3-D to release/7.4

* commit '48a420e0638a23be722cf41eca664cd97a028038':
  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.