documentation

Pull request #2881: feature/ZBXNEXT-9319-7.3-D. real-time widget editing.

Merge in WEB/documentation from feature/ZBXNEXT-9319-7.3-D to release/7.4

* commit 'a4ee99e4cb503d1bec99b32a5af6e8926d1e9b77':
  [ZBXNEXT-9319] fixed shortcomings
  [ZBXNEXT-9319] real-time widget editing
  [ZBXNEXT-9319] improved text for adding/editing/copy-pasting widgets