Was tendermint/tendermint#9930

Tasks:

• an overview of all files that compose the reactor is provided
• all message types exchanged State are described
• all message types exchanged by Gossip are described
• all structures used to keep track of message in Gossip are described
• the interaction between Gossip and State is described
  - A preliminary write up by @cason exists for Tendermint v0.34.x, but should still apply to fork v0.37.x

DoD:

• The current implementation is well understood
  • all components of the current implementation are documented
  • the interaction between Gossip and State is documented.