documentation

Pull request #1670: feature/ZBXNEXT-8908-7.0-D-API. Show values in Pie chart legend (API).

Merge in WEB/documentation from feature/ZBXNEXT-8908-7.0-D-API to release/7.0

* commit '10a5d77fabd706de0672e830c942f65e76cea873':
  [ZBXNEXT-8908] Combined `piechart` widget entries
  Clarified legend parameter behavior
  [ZBXNEXT-8908] Added newline character
  Clarified legend parameter behavior
  [ZBXNEXT-8908] Show values in Pie chart legend (API)