Remove references to frame_consensus from docs

[JoshOrndorff]

Fixes #4574 by removing references to the no-longer-existant frame_consensus.

Should anything more be done here? The docs for Aura used to say:

The Consensus module does not relate directly to Aura, but serves to manage offline reporting by implementing ProvideInherent in a similar way.

[parity-cla-bot]

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Many thanks,

Parity Technologies CLA Bot

[bkchr]

There are more occurrences in this module. However, the whole description of the module in the docs is not correct anymore.

[bkchr]

(maybe not the whole, but crucial parts)

[JoshOrndorff]

I've pushed another commit that attempts a more thorough re-write of the docs for the Session pallet. This writeup is based on my understanding from reading the code and the Seminar about interfacing with Session

While the big-picture ideas are correct, I'm not confident that every detail I've written is correct and would appreciate a thorough review and perhaps mentoring if I missed the target. It seems the docs went stale with #2802

[bkchr]

@JoshOrndorff tests are failing.

[gui1117]

Except the that OnSessionEnding has changed to SessionManager::new_session. Otherwise lgtm