documentation

Pull request #1397: feature/ZBXNEXT-8681-7.0-D. Gauge: better defaults & config form.

Merge in WEB/documentation from feature/ZBXNEXT-8681-7.0-D to release/7.0

* commit '0006f7a22a567e564cdee4598c5b3e7c474702fc':
  [ZBXNEXT-8681] Updated param availability
  [ZBXNEXT-8681] Updated param availability
  [ZBXNEXT-8681] Updated param availability & refined wording
  [ZBXNEXT-8681] Updated param availability
  [ZBXNEXT-8681] Refined wording
  [ZBXNEXT-8681] Gauge: better defaults & config form