Sync docs of procedure with protobuf

[cmungall]

Fixes #382

[cmungall]

@julesjacobsen @pnrobinson I am not sure if this kind of change is permitted while the spec is frozen. My interpretation is that the protobuf is the spec and the documentation can be changed to be in sync with it, but I leave this to you.

[pnrobinson]

In a sane world, the protobuf would be the spec, but that is not the world in which we live in, to quote an immortal beatle. However, I think we have the RTD set up so that there can be multiple versions. I do not think very many people will want to use the ISO specification as the source of truth for implementing phenopackets!