Add documentation about `discard_draft` task in Publishing API
Add documentation about `discard_draft` task in Publishing API

TO DO: When the new discard_draft task in Publishing API is merged, the
`ops:discard` task can be removed.

alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild