Add comment: why activities are stored in redux

dapp/src/store/wallet/walletSlice.ts

@@ -5,6 +5,12 @@ export type WalletState = {
   estimatedBtcBalance: bigint
   sharesBalance: bigint
   isSignedMessage: boolean
+  // Activities are saved in redux because the Acre subgraph may or may not be
+  // up to date with the chain. As a result, if we fetch data from Acre SDK
+  // after the withdrawal is initiated, we may not yet see this activity.
+  // Storing activities in redux gives us the option to add activities manually
+  // by `activityInitialized`. However, fetching the activity from the Acre SDK
+  // again later should already return us the updated data.
   activities: Activity[]
   address: string | undefined
 }