documentation

Pull request #1801: feature/ZBXNEXT-8682-6.4-D. Documented length limit (512KB) for preprocessing test results.

Merge in WEB/documentation from feature/ZBXNEXT-8682-6.4-D to release/6.4

* commit '7ab76366ab02ecde98c360f0a0bd975b7a22649d':
  [ZBXNEXT-8682] fixed feature implementation version
  [ZBXNEXT-8682] documented length limit (512KB) for preprocessing test results