documentation

Pull request #4178: [ZBXNEXT-7502] added new net.if.get agent item

Merge in WEB/documentation from feature/ZBXNEXT-7502-7.5-D to release/8.0

* commit '7c4626401a894c637e01656dbdbc54b18d074aad':
  [ZBXNEXT-7502] fixed typos
  [ZBXNEXT-7502] updated example
  [ZBXNEXT-7502] fixed typo
  [ZBXNEXT-7502] updated description for net.if.get item
  [ZBXNEXT-7502] updated wording
  [ZBXNEXT-7502] updated description for net.if.get agent item
  [ZBXNEXT-7502] added new net.if.get agent item