Add back class documentation for logging.pp
Add back class documentation for logging.pp

This class documentation was added when Heka was introduced. It was
removed when we removed Heka in bd1cb5, but I'm adding it back here as
the class documentation is worth keeping.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet