docs/adr: Update state access ADR to include balance verification (#513)

* docs: include verification against AppHash requirement in state access ADR

* docs: add section for availability of StateServ during sync

docs/adr/adr-004-state-interaction.md

@@ -67,6 +67,19 @@ type StateAccessor interface {
    respond to the queries without relaying queries to celestia-core (*to be scoped out and implemented in later
    iterations*)
 
+### Verification of Balances
+In order to check that the balances returned via the `AccountBalance` query are correct, it is necessary to also request
+Merkle proofs from the celestia-app and verify them against the latest head's `AppHash`. In order for the `StateAccessor`
+to do this, it would need access to the `header.Store`'s `Head()` method in order to get the latest known header of the 
+node and check its `AppHash`.
+
+### Availability of `StateService` during sync
+The `Syncer` in the `header`  package provides one public method, `Finished()`, that indicates whether the syncer has 
+finished syncing. Introducing the availability of `StateService` would require extending the public API for `Syncer` 
+with an additional method, `NetworkHead()`, in order to be able to fetch *current* state from the network. The `Syncer`
+would then have to be passed to any implementation of `StateService` upon construction and relied on in order to access 
+the network head even if the syncer is still syncing, as the network head is still verified even during sync.
+
 ### 1. Core Implementation of `StateAccessor`: `CoreAccess`
 
 `CoreAccess` will be the direct RPC connection implementation of `StateAccessor`. It will maintain a gRPC connection