docs(signals): add info about throwing error on state mutation in dev mode (#4595)

Co-authored-by: Marko Stanimirović <markostanimirovic95@gmail.com>

Author: rainerhahnekamp

projects/ngrx.io/content/guide/signals/signal-state.md

@@ -87,6 +87,7 @@ patchState(
 <div class="alert is-critical">
 
 Updaters passed to the `patchState` function must perform state updates in an immutable manner.
+If a mutable change occurs to the state object, an error will be thrown in development mode.
 
 </div>