Review and update RPC help messages of new sc_ commands

[cronicc]

E.g. for the sc_create command we need to go into more detail about the fields actually mean. Here is an example of how it looks currently, which doesn't provide useful info to the user:

 "wCertVk":data                  (string, required) It is an arbitrary byte string of even length expressed in
                                       hexadecimal format. Required to verify a WCert SC proof. Its size must be 9216 bytes max