docs: update app-architecture.md (#4259)

Address #4089 by clarifying the different uses of "application"

docs/app-dev/app-architecture.md

@@ -11,26 +11,33 @@ The following diagram provides a superb example:
 
 ![](../imgs/cosmos-tendermint-stack-4k.jpg)
 
-The end-user application here is the Cosmos Voyager, at the bottom left.
-Voyager communicates with a REST API exposed by a local Light-Client
+We distinguish here between two forms of "application". The first is the
+end-user application, like a desktop-based wallet app that a user downloads,
+which is where the user actually interacts with the system. The other is the
+ABCI application, which is the logic that actually runs on the blockchain.
+Transactions sent by an end-user application are ultimately processed by the ABCI
+application after being committed by the Tendermint consensus.
+
+The end-user application in this diagram is the Cosmos Voyager, at the bottom
+left. Voyager communicates with a REST API exposed by a local Light-Client
 Daemon. The Light-Client Daemon is an application specific program that
-communicates with Tendermint nodes and verifies Tendermint light-client
-proofs through the Tendermint Core RPC. The Tendermint Core process
-communicates with a local ABCI application, where the user query or
-transaction is actually processed.
+communicates with Tendermint nodes and verifies Tendermint light-client proofs
+through the Tendermint Core RPC. The Tendermint Core process communicates with
+a local ABCI application, where the user query or transaction is actually
+processed.
 
 The ABCI application must be a deterministic result of the Tendermint
 consensus - any external influence on the application state that didn't
 come through Tendermint could cause a consensus failure. Thus _nothing_
-should communicate with the application except Tendermint via ABCI.
+should communicate with the ABCI application except Tendermint via ABCI.
 
-If the application is written in Go, it can be compiled into the
+If the ABCI application is written in Go, it can be compiled into the
 Tendermint binary. Otherwise, it should use a unix socket to communicate
 with Tendermint. If it's necessary to use TCP, extra care must be taken
 to encrypt and authenticate the connection.
 
-All reads from the app happen through the Tendermint `/abci_query`
-endpoint. All writes to the app happen through the Tendermint
+All reads from the ABCI application happen through the Tendermint `/abci_query`
+endpoint. All writes to the ABCI application happen through the Tendermint
 `/broadcast_tx_*` endpoints.
 
 The Light-Client Daemon is what provides light clients (end users) with
@@ -41,7 +48,7 @@ be implemented in the same process as the end-user application.
 
 Note for those ABCI applications with weaker security requirements, the
 functionality of the Light-Client Daemon can be moved into the ABCI
-application process itself. That said, exposing the application process
+application process itself. That said, exposing the ABCI application process
 to anything besides Tendermint over ABCI requires extreme caution, as
 all transactions, and possibly all queries, should still pass through
 Tendermint.