Rewrite Share interface docs and split into two APIs.

docs/explainer.md

 # Ballista Explained
 
 **Date**: 2015-09-25
 
 **Ballista** is a project to explore inter-website communication; specifically,
 communication between one website and another site of the user's choosing.
 Imagine being able to:
 
 * Click a "share" button, then choose which social network or other web/native
   app to share it with, based on which apps *you* have installed (not a
@@ skipping 96 matching lines @@
 
 To let the user share the current page's URL with an app or website of their
 choosing, just attach this JavaScript code to a "share" button.
 
 > **Note:** Only one-way actions can be requested from a foreground page.
 
 #### foreground.js
 
 ```js
 shareButton.addEventListener('click', () => {
-  navigator.actions.performAction('share', {url: window.location.href})
-      .then(action => console.log(action));
+  navigator.share({url: window.location.href})
+      .then(console.log('Share successful'));
 });
 ```
 
 #### User experience
 
 1. The user clicks the "share" button. The browser shows a list of registered
    share handlers, and the user can pick one.
 
 ### Share handler
 
 Here's how to register a website to appear in the list of apps that can handle a
 "share" intent on Android, or a "share" action from another website.
 
 You need both a [web app manifest](https://w3c.github.io/manifest/) and a
 [service
 worker](http://slightlyoff.github.io/ServiceWorker/spec/service_worker/),
 so that your site can be contacted even when the user does not have it open in
 any tabs.
 
 #### manifest.webmanifest
 
 ```JSON
 {
   "name": "Includinator",
   "short_name": "Includinator",
   "icons": [...],
-  "actions": [
-    {
-      "verb": "share"
-    }
-  ]
+  "supports_share": true
 }
 ```
 
 #### serviceworker.js
 
 ```js
-navigator.actions.addEventListener('handle', event => {
-  if (event.options.verb == 'share') {
-    if (event.data.url === undefined)
-      throw new Error('Did not contain URL.');
+navigator.actions.addEventListener('share', event => {
+  if (event.data.url === undefined)
+    throw new Error('Did not contain URL.');
 
-    includinate(event.data.url);
-  }
+  includinate(event.data.url);
 });
 ```
 
 #### User experience
 
 1. When the user is on your site, the browser provides a button to register the
    app as a "share handler".
 2. The user clicks this button. The app is registered and appears in the list of
    share handlers that the browser shows to the user.
 
@@ skipping 108 matching lines @@
    app as an editor for text files.
 2. The user clicks this button. The app is registered and appears in the list of
    text file editors that the browser shows to the user. It is also registered
    as a file association for `*.txt` files in the host OS.
 3. When the user opens a text file from another website, the browser lets them
    pick your app. Or, the user can right-click on a text file in their local
    file browser and choose to open it with your app. Either way, this pops open
    your app in a new browser tab.
 4. When the user clicks "Save" in your app, the updated file is passed back to
    the requester website, or written back to disk.