Improve the documentation for generated Stub functions

[davydog187]

Is your feature request related to a problem? Please describe.
Functions generated via use GRPC.Stub, service: MyService are not well documented. Understanding the arity and shape of expected parameters require digging into the source code to find GRPC.Stub.call/6 information. The docs currently make a vague mention to this

Describe the solution you'd like
Document how injected functions are generated so that it is easy to find

Describe alternatives you've considered
N/A

Additional context
N/A

[davydog187]

I have spoken to @polvalente about this, creating an issue first in case someone else gets to it before me. I'm planning on sending a PR