documentation

Pull request #567: ZBX-20320 6.4 Updated User roles table and description

Merge in WEB/documentation from feature/ZBX-20320-6.4-D to release/6.4

* commit '1158e1a3696b979313ba98c29406d1876a0cfd92':
  ZBX-20320 Deleted double User roles page.
  ZBX-20320 Edited Markdown syntax on User roles.
  ZBX-20320 Updated User roles page.