Better Group docs. Fix phaserjs#5011

Author: photonstorm

src/physics/arcade/PhysicsGroup.js

@@ -15,10 +15,19 @@ var IsPlainObject = require('../../utils/object/IsPlainObject');
  * @classdesc
  * An Arcade Physics Group object.
  *
- * All Game Objects created by or added to this Group will automatically be given dynamic Arcade Physics bodies (if they have no body)
- * and the bodies will receive the Group's {@link Phaser.Physics.Arcade.Group#defaults default values}.
+ * The primary use of a Physics Group is a way to collect together physics enable objects
+ * that share the same intrinsic structure into a single pool. They can they be easily
+ * compared against other Groups, or Game Objects.
  *
- * Its static counterpart is {@link Phaser.Physics.Arcade.StaticGroup}.
+ * All Game Objects created by, or added to this Group will automatically be given **dynamic**
+ * Arcade Physics bodies (if they have no body already) and the bodies will receive the
+ * Groups {@link Phaser.Physics.Arcade.Group#defaults default values}.
+ *
+ * You should not pass objects into this Group that should not receive a body. For example,
+ * do not add basic Geometry or Tilemap Layers into a Group, as they will not behave in the
+ * way you may expect. Groups should all ideally have objects of the same type in them.
+ *
+ * If you wish to create a Group filled with Static Bodies, please see {@link Phaser.Physics.Arcade.StaticGroup}.
  *
  * @class Group
  * @extends Phaser.GameObjects.Group