Please document or at least explain where/how to find which additional parameters can be added to the Generic SCM step for a specific SCM plugin.

E.g. it would be useful to set the `workspaceUpdater: [$class: 'UpdateWithCleanUpdater']` for an SVN checkout in a Generic SCM step.

Or at least to have a clear indication that setting this parameter (as an example) is only possible using the specific SVN step.