doc: Add release notes for changes in RPCs

Adding notes for both `unloadwallet` and `getdescriptoractivity`.

doc/release-notes-32845.md

@@ -0,0 +1,9 @@
+Updated RPCs
+------------
+
+- `unloadwallet` - Return RPC_INVALID_PARAMETER when both the RPC wallet endpoint
+and wallet_name parameters are unspecified. Previously the RPC failed with a JSON
+parsing error.
+- `getdescriptoractivity` - Mark blockhashes and scanobjects arguments as required,
+so the user receives a clear help message when either is missing. As in `unloadwallet`,
+previously the RPC failed with a JSON parsing error.