documentation

Pull request #1474: feature/ZBX-23235-7.0-D. Added note on value mapping substitution (w/ page wording & formatting improvements).

Merge in WEB/documentation from feature/ZBX-23235-7.0-D to release/7.0

* commit 'b706fcd65d3ee6586464ff603f53c477c755d474':
  [ZBX-23235] Updated wording & formatting
  [ZBX-23235] Restored table formatting
  [ZBX-23235] Made note more symmetric
  [ZBX-23235] Added note on value mapping substitution
  [ZBX-23235] Improved page (wording, formatting, structure)