docs: update store recipes to clarify proper use of META_REDUCERS tok…

…en in libs (#1970)

projects/ngrx.io/content/guide/store/recipes/injecting.md

@@ -77,12 +77,23 @@ export function getMetaReducers(
       provide: META_REDUCERS,
       deps: [SomeService],
       useFactory: getMetaReducers,
+      multi: true,
     },
   ],
 })
 export class AppModule {}
 </code-example>
 
+<div class="alert is-important">
+
+Careful attention should be called to the use of the `multi` 
+property in the provider here for `META_REDUCERS`. As this injection token may be utilized 
+by many libraries concurrently, specifying `multi: true` is critical to ensuring that all 
+library meta reducers are applied to any project that consumes multiple NgRx libraries with 
+registered meta reducers.
+
+</div>
+
 
 ## Injecting Feature Config