docs(operators): add documentation for pairwise

src/operator-docs/combination/pairwise.ts

@@ -1,6 +1,51 @@
 import { OperatorDoc } from '../operator.model';
 
 export const pairwise: OperatorDoc = {
-  'name': 'pairwise',
-  'operatorType': 'combination'
+  name: 'pairwise',
+  operatorType: 'combination',
+  marbleUrl: 'http://reactivex.io/rxjs/img/pairwise.png',
+  signature: 'public pairwise(): Observable<Array<T>>',
+  shortDescription: {
+    description:
+      'Groups pairs of consecutive emissions together and emits them as an array of two values.',
+    extras: [
+      {
+        type: 'Tip',
+        text:
+          'Puts the current value and previous value together as an array, and emits that.'
+      }
+    ]
+  },
+  walkthrough: {
+    description: `
+      <p>The Nth emission from the source Observable will cause the output Observable
+      to emit an array [(N-1)th, Nth] of the previous and the current value, as a
+      pair. For this reason, <code>pairwise</code> emits on the second and subsequent
+      emissions from the source Observable, but not on the first emission, because
+      there is no previous value in that case.</p>
+    `
+  },
+  examples: [
+    {
+      name:
+        'On every click (starting from the second), emit the relative distance to the previous click',
+      code: `
+      const clicks = Rx.Observable.fromEvent(document, 'click');
+      const pairs = clicks.pairwise();
+      const distance = pairs.map(pair => {
+        const x0 = pair[0].clientX;
+        const y0 = pair[0].clientY;
+        const x1 = pair[1].clientX;
+        const y1 = pair[1].clientY;
+        return Math.sqrt(Math.pow(x0 - x1, 2) + Math.pow(y0 - y1, 2));
+      });
+      distance.subscribe(x => console.log(x));
+    `,
+      externalLink: {
+        platform: 'JSBin',
+        url: 'http://jsbin.com/wenazagegu/embed?js,console,output'
+      }
+    }
+  ],
+  relatedOperators: ['buffer', 'bufferCount']
 };