documentation

Pull request #1794: feature/ZBXNEXT-8682-7.0-D. Documented length limit (512KB) for item and preprocessing test results and values retrieved from hosts.

Merge in WEB/documentation from feature/ZBXNEXT-8682-7.0-D to release/7.0

* commit '910ff21731abeb176f11e248407e64a3de63747b':
  [ZBXNEXT-8682] updated wording
  [ZBXNEXT-8682] fixed link
  [ZBXNEXT-8682] documented length limit (512KB) for item and preprocessing test results and values retrieved from hosts