Add note about the store retaining the query watcher

Sources/Apollo/GraphQLQueryWatcher.swift

@@ -1,6 +1,8 @@
 import Dispatch
 
 /// A `GraphQLQueryWatcher` is responsible for watching the store, and calling the result handler with a new result whenever any of the data the previous result depends on changes.
+///
+/// NOTE: The store retains the watcher while subscribed. You must call `cancel()` on your query watcher when you no longer need results. Failure to call `cancel()` before releasing your reference to the returned watcher will result in a memory leak.
 public final class GraphQLQueryWatcher<Query: GraphQLQuery>: Cancellable, ApolloStoreSubscriber {
   weak var client: ApolloClientProtocol?
   public let query: Query

docs/source/caching.md

@@ -53,7 +53,7 @@ let watcher = apollo.watch(query: HeroNameQuery(episode: .empire)) { result in
 }
 ```
 
-> Remember to kill off watchers when their parent object is deallocated! This is not (presently) done automatically.
+> **NOTE:** Remember to call `cancel()` on a watcher when its parent object is deallocated, or you will get a memory leak! This is not (presently) done automatically.
 
 ## Direct cache access