documentation

Pull request #379: [ZBXNEXT-7661] Updated "What's new" and "Item value" pages.

Merge in WEB/documentation from feature/ZBXNEXT-7661-6.4-D to release/6.4

* commit '3d9089bea0875fbb87511ca62f89fe073d8b4395':
  item_value.md edited online with Bitbucket
  [ZBXNEXT-7661] Updated description for thresholds
  [ZBXNEXT-7661] Updated "What's new" page and thresholds description
  [ZBXNEXT-7661] Updated description for thresholds
  [ZBXNEXT-7661] Updated grammar and markdown
  [ZBXNEXT-7661] Updated "What's new" and "Item value" widget page