documentation

Pull request #766: Feature/ZBXNEXT-8074 6.4 D

Merge in WEB/documentation from feature/ZBXNEXT-8074-6.4-D to release/6.4

* commit '945aa8f9e3131ca054af4a3d484f2dcafee8f9df':
  [ZBXNEXT-8074] added secret property to user.checkauthentication
  [ZBXNEXT-8074] reprased CSRF tokens.
  [ZBXNEXT-8074] rephrased CSRF Tokens
  upgrade_notes_640.md formatting fix
  [ZBXNEXT-8074] updated CSRF tokens
  formatting fix
  [ZBXNEXT-8074] added CSRF token note