documentation

Pull request #422: ZBXNEXT-7931 Updated on heartbeat drop on 6 p.

Merge in WEB/documentation from feature/ZBXNEXT-7931-6.4-D to release/6.4

* commit '47ef2a1cda864179627760c1e69c34231ce50a43':
  ZBXNEXT-7931 updated manpages
  ZBXNEXT-7931 Edited Upgrade notes and Whatsnew p.
  ZBXNEXT-7931 Edited Upgrade and Whatsnew pages.
  ZBXNEXT-7931 Corrected grammar in 3p.
  ZBXNEXT-7931 Edited heartbeat drop on 4p.
  ZBXNEXT-7931 Updated on heartbeat drop on 6 p.