GlobalKey docs improvement (#66834)

Improve GlobalKey docs by mentioning a pitfall that some users had where the instantiated it in the build method.

packages/flutter/lib/src/widgets/framework.dart

@@ -116,6 +116,20 @@ class ObjectKey extends LocalKey {
 /// You cannot simultaneously include two widgets in the tree with the same
 /// global key. Attempting to do so will assert at runtime.
 ///
+/// ## Pitfalls
+/// GlobalKeys should not be re-created on every build. They should usually be
+/// long-lived objects owned by a [State] object, for example.
+///
+/// Creating a new GlobalKey on every build will throw away the state of the
+/// subtree associated with the old key and create a new fresh subtree for the
+/// new key. Besides harming performance, this can also cause unexpected
+/// behavior in widgets in the subtree. For example, a [GestureDetector] in the
+/// subtree will be unable to track ongoing gestures since it will be recreated
+/// on each build.
+///
+/// Instead, a good practice is to let a State object own the GlobalKey, and
+/// instantiate it outside the build method, such as in [State.initState].
+///
 /// See also:
 ///
 ///  * The discussion at [Widget.key] for more information about how widgets use