documentation

Pull request #1597: feature/ZBXNEXT-8145-7.0-D2-API. Added "Reference" parameter (for linking widgets).

Merge in WEB/documentation from feature/ZBXNEXT-8145-7.0-D2-API to release/7.0

* commit 'eb54c183e911abd873e63ac0408927432bc3a170':
  [ZBXNEXT-8145] Updated Reference parameter description, and widget examples
  [ZBXNEXT-8145] Clarified Reference parameter description
  [ZBXNEXT-8145] Updated wording and formatting in parameter description
  [ZBXNEXT-8145] Added "Reference" parameter (for linking widgets)