documentation

Pull request #1666: feature/ZBX-23972-7.0-D-API. Added note on widget field validation.

Merge in WEB/documentation from feature/ZBX-23972-7.0-D-API to release/7.0

* commit '7151e180386607d777b1b6ca2c93a854f4d4e3ef':
  [ZBX-23972] Added note on widget field validation