Support for Live Data description

The Live Data macro has a description parameter, allowing to define a textual description of the content of the Live Data.

Example:

{{liveData
  id="users"
  properties="_avatar,doc.name,first_name,last_name"
  source="liveTable"
  sourceParameters="className=XWiki.XWikiUsers&translationPrefix=xe.userdirectory."
  description="The list of all users:"
/}}