documentation

Pull request #1858: feature/ZBX-24301-7.0-D. Updated net.dns item return values description & added version-specific info to net.dns.perf item.

Merge in WEB/documentation from feature/ZBX-24301-7.0-D to release/7.0

* commit '28fd6dddfde46a02951832f67c5a72916aa40e09':
  [ZBX-24301] refined wording by mentioning error code examples
  [ZBX-24301] refined wording
  [ZBX-24301] added version-specific info to net.dns.perf item
  [ZBX-24301] added feature-related "What's new" and "Upgrade notes" entry
  [ZBX-24301] updated net.dns item description
  [ZBX-24301] updated net.dns item return values description
  fixed link