Documenting send_input and receive_input

[abrookins]

Expands current docs on interactive flows and includes details on using send_input and receive_input.

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[discdiver]

Thank you! Very thorough!

I think it's worth adding somewhere that these example work without async syntax (I know some do at least).

A number of minor nits and fixes.

[abrookins]

@discdiver Ok, I'm finally done with this, I think!