docs(Observable): add documentation for Observable and operators

Closes #5642
Closes #5684

modules/angular2/examples/facade/ts/async/observable_patched.ts

@@ -0,0 +1,10 @@
+// #docregion Observable
+import {Observable} from 'angular2/core';
+import 'rxjs/add/operator/map';
+
+var obs = new Observable(obs => {
+  var i = 0;
+  setInterval(_ => obs.next(++i), 1000);
+});
+obs.map(i => `${i} seconds elapsed`).subscribe(msg => console.log(msg));
+// #enddocregion