documentation

Pull request #2586: ZBX-26215 clarified what media types will send a notification if DB is down.

Merge in WEB/documentation from feature/ZBX-26215-7.0-D to release/7.0

* commit '3f5df1a69e5e1c618a19fc52abe9aa7c93911f8d':
  ZBX-26215 improved description readability.
  ZBX-26215 clarified what media types will send a notification if DB is down.