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.