Document the 'type' parameter for client registration.

The pump.io API documentation says client registration is loosely modelled on the OpenID Dynamic Client Registration specification, still under development, and apparently it includes a required parameter named 'type'. Looking at the current (2013-08-26) version of the OpenID DCR draft spec, it seems that since the pump.io API was drafted, DCR has (a) renamed the 'type' parameter to 'operation' (in revision 14) and (b) deleted the 'operation' parameter (in revision 15), leaving no documentation on what pump.io actually expects this field to contain.