documentation

Pull request #2163: feature/ZBX-21543-7.2-D. clarified "Check for not supported value" preprocessing step behavior.

Merge in WEB/documentation from feature/ZBX-21543-7.2-D to release/7.2

* commit '260e1d3e81c7c9d88069ab5605c7b68e439cb6ed':
  [ZBX-21543] fixed heading size; added missing "template"
  [ZBX-21543] clarified "Check for not supported value" preprocessing step behavior
  [ZBX-21543] improved "Overview" structure
  [ZBX-21543] improved "Overview" wording
  [ZBX-21543] fixed regex examples
  [ZBX-21543] updated structure, wording, formatting
  [ZBX-21543] fixed line breaks