documentation

Pull request #1487: feature/ZBXNEXT-5401-7.0-D. Added net.dns.perf agent item.

Merge in WEB/documentation from feature/ZBXNEXT-5401-7.0-D to release/7.0

* commit '3a922c7a60340e26ef9222f1b0f4792675fcd1e9':
  [ZBXNEXT-5401] Updated description for net.*.perf agent items
  [ZBXNEXT-5401] Updated description for net.dns.perf agent item
  [ZBXNEXT-5401] Updated "Items" section formatting
  [ZBXNEXT-5401] Fixed punctuation
  [ZBXNEXT-5401] Added net.dns.perf agent item