fixes #5780 - API v2 - RESTful way to add/remove many-to-many association one at a time rather than passing all id's

Included commit from PR #1068
@ohadlevy, @domcleal, Should I close this PR (to add/remove many-to-many association one at a time) if we are going to add `location_ids`, `organization_ids` and other *_ids to the documentation as the means to add/remove associations.