[protocolv2] always expect init, input sometimes (docs only)

[masad-frost]

Why

Currently we expect:

• 0 init messages for rpc and subscription, and 0-1 init messages for upload and stream
• 1-n input messages for upload and stream, and 0 input for rpc and subscription

This works but if we did:

• 1 init message for all
• 0-n input messages (always 0 for rpc and subscription)

We'd have a shape of streams that is a little more consistent, making implementations simpler, although practically it's not much simpler since it's papered over in the libraries.

What changed

Updated documentation for init and input.

[masad-frost]

In practice this doesn't affect the implementations since wire messages don't differentiate between init and input.