Here is a template for documenting services and service capabilities as part of building up a service/domain inventory (it is available from the resources page also). This was inspired by the items from the book Principles of Service Design. I modified this slightly to include a few additional fields. Feel free to modify this template to suit your needs.
A collection of these documents can be managed via a Wiki or document management system as a starting point. As your SOA efforts mature you would want to consider replacing much of the technical details here with a governance tool. These tools can synch up schema, WSDL changes, WS-Policy specifications etc and have the ability to generate service dashboards and documentation. You might most likely still need a document to capture policies specific to your organization or SLA characteristics not captured via governance tools etc.