documentation

Pull request #2370: ZBX-21821 added a note for recommended expression for the widget parameters.

Merge in WEB/documentation from feature/ZBX-21821-7.0-D to release/7.0

* commit 'fab8fffca6e55ba17e7ae7b88c7e0249ba493006':
  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.