Document custom-style module property.

src/lib/custom-style.html

@@ -23,6 +23,9 @@
 browsers without non-native Shadow DOM.
 * Custom properties used by Polymer's shim for cross-scope styling
 may be defined in an `custom-style`.
+* A module attribute may be specified to pull in style data from a `dom-module`
+matching the module attribute. By using `module`, style data
+can be shared between multiple `custom-style` elements.
 
 Example:
 
@@ -82,6 +85,11 @@
 
     properties: {
       // module is a property so that it deserializes
+      /**
+       * Specify module to identify a `dom-module` containing style data which
+       * should be used within the `custom-style`. By using `module` style data
+       * may be shared between multiple different `custom-style` elements.
+       */
       module: String
     },