docs(Symbol.observable): improve docs around observable interop method as per request

Author: benlesh

src/createStore.js

@@ -200,14 +200,22 @@ export default function createStore(reducer, initialState, enhancer) {
   }
 
   /**
-   * Interop point for observable libraries
-   * @returns {observable} minimal observable of state changes. This was added for Interop
-   * For more information, see the
-   * [observable proposal](https://github.com/zenparsing/es-observable)
+   * Interoperability point for observable/reactive libraries.
+   * @returns {observable} A minimal observable of state changes.
+   * For more information, see the observable proposal:
+   * https://github.com/zenparsing/es-observable
    */
   function observable() {
     var outerSubscribe = subscribe
     return {
+      /**
+       * The minimal observable subscription method.
+       * @param {Object} observer Any object that can be used as an observer.
+       * The observer object should have a `next` method.
+       * @returns {subscription} An object with an `unsubscribe` method that can
+       * be used to unsubscribe the observable from the store, and prevent further
+       * emission of values from the observable.
+       */
       subscribe(observer) {
         if (typeof observer !== 'object') {
           throw new TypeError('Expected observer to be an object')