The Risk Management Plan is one document of many that combine to make the Project Management Plan, in my view. The Project Management Plan is a set of documents that tell you how the project will be managed in terms of scope, cost, quality, change, resource, risks and much more.
The Risk Management Plan defines how risks will be controlled within the project. It begins with the identification and then defines the action plan to deal with each type of risk. The Risk Management Plan does not identify individual risks, but is a structure for dealing with risks.