documentation

Pull request #4130: ZBXNEXT-6492 updated parameters description, added a what's new entry, added info for PostgreSQL during frontend installation.

Merge in WEB/documentation from feature/ZBXNEXT-6492-7.5-D to release/8.0

* commit '4030bde985480f890c0daa28d7f6f361e03cf5fc':
  ZBXNEXT-6492 copied DBhost description from server to proxy config file.
  ZBXNEXT-6492 clarifications and improvements.
  ZBXNEXT-6492 updated parameters description, added a what's new entry, added info for PostgreSQL during frontend installation.