Clarify all elements between changes must apply mixing.

Fixes #4914

lib/legacy/mutable-data-behavior.html

@@ -42,6 +42,9 @@
    * (e.g. `this.notifyPath('items')`) to update the tree.  Note that all
    * elements that wish to be updated based on deep mutations must apply this
    * mixin or otherwise skip strict dirty checking for objects/arrays.
+   * Specifically, any elements in the binding tree between the source of a
+   * mutation and the consumption of it must apply this behavior or enable the
+   * `Polymer.OptionalMutableDataBehavior`.
    *
    * In order to make the dirty check strategy configurable, see
    * `Polymer.OptionalMutableDataBehavior`.
@@ -102,6 +105,9 @@
    * (e.g. `this.notifyPath('items')`) to update the tree.  Note that all
    * elements that wish to be updated based on deep mutations must apply this
    * mixin or otherwise skip strict dirty checking for objects/arrays.
+   * Specifically, any elements in the binding tree between the source of a
+   * mutation and the consumption of it must enable this behavior or apply the
+   * `Polymer.OptionalMutableDataBehavior`.
    *
    * While this behavior adds the ability to forgo Object/Array dirty checking,
    * the `mutableData` flag defaults to false and must be set on the instance.

lib/mixins/mutable-data.html

@@ -57,6 +57,9 @@
    * (e.g. `this.notifyPath('items')`) to update the tree.  Note that all
    * elements that wish to be updated based on deep mutations must apply this
    * mixin or otherwise skip strict dirty checking for objects/arrays.
+   * Specifically, any elements in the binding tree between the source of a
+   * mutation and the consumption of it must apply this mixin or enable the
+   * `Polymer.OptionalMutableData` mixin.
    *
    * In order to make the dirty check strategy configurable, see
    * `Polymer.OptionalMutableData`.
@@ -132,6 +135,9 @@
    * (e.g. `this.notifyPath('items')`) to update the tree.  Note that all
    * elements that wish to be updated based on deep mutations must apply this
    * mixin or otherwise skip strict dirty checking for objects/arrays.
+   * Specifically, any elements in the binding tree between the source of a
+   * mutation and the consumption of it must enable this mixin or apply the
+   * `Polymer.MutableData` mixin.
    *
    * While this mixin adds the ability to forgo Object/Array dirty checking,
    * the `mutableData` flag defaults to false and must be set on the instance.