953665 - added ability for copy commands to specify the fields of their units that should be fetched, so as to avoid loading the entirety of every unit in the source repository into RAM. Also added the ability to provide a custom "override_config" based on CLI options.
adding documentation
Merge branch 'mhrivnak-953665'

Conflicts:
	platform/src/pulp/bindings/repository.py
	platform/test/unit/server/test_repo_unit_association_manager.py