docs(pipes): add PipeTransform interface & comment

closes #616

public/docs/_examples/pipes/ts/app/exponential-strength.pipe.ts

@@ -1,5 +1,5 @@
 // #docregion
-import {Pipe} from 'angular2/core';
+import {Pipe, PipeTransform} from 'angular2/core';
 /*
  * Raise the value exponentially
  * Takes an exponent argument that defaults to 1.
@@ -10,7 +10,7 @@ import {Pipe} from 'angular2/core';
  *   formats to: 1024
 */
 @Pipe({name: 'exponentialStrength'})
-export class ExponentialStrengthPipe {
+export class ExponentialStrengthPipe implements PipeTransform {
 
   transform(value:number, args:string[]) : any {
     return Math.pow(value, parseInt(args[0] || '1', 10));

public/docs/_examples/pipes/ts/app/fetch-json.pipe.ts

@@ -1,6 +1,6 @@
 /// <reference path='./window.extension.d.ts'/>
 // #docregion
-import {Pipe} from 'angular2/core';
+import {Pipe, PipeTransform} from 'angular2/core';
 
 // #docregion pipe-metadata
 @Pipe({
@@ -8,7 +8,7 @@ import {Pipe} from 'angular2/core';
   pure: false
 })
 // #enddocregion pipe-metadata
-export class FetchJsonPipe {
+export class FetchJsonPipe  implements PipeTransform{
   private fetchedValue:any;
   private fetchPromise:Promise<any>;
 

public/docs/ts/latest/guide/pipes.jade

@@ -124,7 +124,7 @@ figure.image-display
   
   * A pipe is a class decorated with pipe metadata.
   
-  * The pipe class implements a `transform` method that
+  * The pipe class implements the `PipeTransform` interface's `transform` method that
   takes an input value and an optional array of parameter strings and returns the transformed value.
 
   * There will be one item in the parameter array for each parameter passed to the pipe
@@ -136,7 +136,15 @@ figure.image-display
      pipe name that we'll use within a template expression. It must be a valid JavaScript identifier.
      Our pipe's name is `exponenentialStrength`.
 
+.l-sub-section
+  :marked
+    ### The *PipeTransform* Interface
 
+    The `transform` method is essential to a pipe. 
+    The `PipeTransform` interface defines that method and guides both tooling and the compiler.
+    It is optional; Angular looks for and executes the `transform` method regardless.
+    
+:marked
   Now we need a component to demonstrate our pipe.
 +makeExample('pipes/ts/app/power-booster.component.ts',null,'app/power-booster.component.ts')
 figure.image-display