expand validity subsystems descriptions independent of subsystems overview (#1185)

* expand subsystems descriptions independent of subsystems overview

The subsystems overview in https://github.com/paritytech/polkadot/pull/1161
is a very useful reference for what subsystems need to talk to each other,
when, and why; it helps us design the actual messages.

However, administratively, it belongs in a different PR. This commit
brings in all the changes made there so far as a base for an independent
PR.

* Reorder subsystem descriptions, add some messages

Update ordering of subsystem descriptions to rough order of use,
mirroring the order in the overview document.

Added some message types. Added OverseerSignal variants to several
types, such that each subsystem only needs to listen for a single
type.

* add some more message types, Statement Distribution overview

* add more detail on Statement Distribution, Misbehavior Arbitration

* intentionally punt MA details for a future PR

* reduce duplication from overseer signal handling

* reword for clarity

* clarify: other modules and subsystems also get to talk to the network

* finish current work on candidate selection

* update candidate backing subsystem description according to current thought

* update mechanism for candidate backing to report collator misbehavior to candidate selection

* sketch out the peer receipt state machine

* Fix typo in roadmap/implementors-guide/guide.md

Co-authored-by: Robert Habermeier <[email protected]>

* Don't specify 'peer validators', as messages from non-validator peers are ignored regardless

* clarify instancing of peer receipt state machine

* add section on peer knowledge tracking

* fix typo in roadmap/implementors-guide/guide.md

Co-authored-by: Max Inden <[email protected]>

Co-authored-by: Robert Habermeier <[email protected]>
Co-authored-by: Max Inden <[email protected]>