chore(docs): tidy Puppeteer class docs (#6181)

* Mark properties as `private` or `@internal` so they don't get
  documented.
* Fix the `@returns` on a getter which gets ignored.

new-docs/puppeteer.md

@@ -27,7 +27,7 @@
 |  [Keyboard](./puppeteer.keyboard.md) | Keyboard provides an api for managing a virtual keyboard. The high level api is [Keyboard.type()](./puppeteer.keyboard.type.md)<!-- -->, which takes raw characters and generates proper keydown, keypress/input, and keyup events on your page. |
 |  [Mouse](./puppeteer.mouse.md) | The Mouse class operates in main-frame CSS pixels relative to the top-left corner of the viewport. |
 |  [Page](./puppeteer.page.md) | Page provides methods to interact with a single tab or [extension background page](https://developer.chrome.com/extensions/background_pages) in Chromium. |
-|  [Puppeteer](./puppeteer.puppeteer.md) | The main Puppeteer class Puppeteer module provides a method to launch a browser instance. |
+|  [Puppeteer](./puppeteer.puppeteer.md) | The main Puppeteer class. Provides the [launch](./puppeteer.puppeteer.launch.md) method to launch a browser.<!-- -->When you <code>require</code> or <code>import</code> the Puppeteer npm package you get back an instance of this class. |
 |  [SecurityDetails](./puppeteer.securitydetails.md) | The SecurityDetails class represents the security details of a response that was received over a secure connection. |
 |  [Target](./puppeteer.target.md) |  |
 |  [TimeoutError](./puppeteer.timeouterror.md) | TimeoutError is emitted whenever certain operations are terminated due to timeout. |

new-docs/puppeteer.puppeteer.md

@@ -4,7 +4,9 @@
 
 ## Puppeteer class
 
-The main Puppeteer class Puppeteer module provides a method to launch a browser instance.
+The main Puppeteer class. Provides the [launch](./puppeteer.puppeteer.launch.md) method to launch a browser.
+
+When you `require` or `import` the Puppeteer npm package you get back an instance of this class.
 
 <b>Signature:</b>
 
@@ -37,13 +39,9 @@ const puppeteer = require('puppeteer');
 
 |  Property | Modifiers | Type | Description |
 |  --- | --- | --- | --- |
-|  [\_\_productName](./puppeteer.puppeteer.__productname.md) |  | string |  |
-|  [\_changedProduct](./puppeteer.puppeteer._changedproduct.md) |  | boolean |  |
-|  [\_isPuppeteerCore](./puppeteer.puppeteer._ispuppeteercore.md) |  | boolean |  |
-|  [\_preferredRevision](./puppeteer.puppeteer._preferredrevision.md) |  | string |  |
 |  [devices](./puppeteer.puppeteer.devices.md) |  | [DevicesMap](./puppeteer.devicesmap.md) |  |
 |  [errors](./puppeteer.puppeteer.errors.md) |  | [PuppeteerErrors](./puppeteer.puppeteererrors.md) |  |
-|  [product](./puppeteer.puppeteer.product.md) |  | string |  |
+|  [product](./puppeteer.puppeteer.product.md) |  | string | The name of the browser that is under automation (<code>&quot;chrome&quot;</code> or <code>&quot;firefox&quot;</code>) |
 
 ## Methods
 

new-docs/puppeteer.puppeteer.product.md

@@ -4,6 +4,8 @@
 
 ## Puppeteer.product property
 
+The name of the browser that is under automation (`"chrome"` or `"firefox"`<!-- -->)
+
 <b>Signature:</b>
 
 ```typescript

src/common/Puppeteer.ts

@@ -35,8 +35,11 @@ import {
 import { PUPPETEER_REVISIONS } from '../revisions';
 
 /**
- * The main Puppeteer class
- * Puppeteer module provides a method to launch a browser instance.
+ * The main Puppeteer class. Provides the {@link Puppeteer.launch | launch}
+ * method to launch a browser.
+ *
+ * When you `require` or `import` the Puppeteer npm package you get back an
+ * instance of this class.
  *
  * @remarks
  *
@@ -57,11 +60,14 @@ import { PUPPETEER_REVISIONS } from '../revisions';
  */
 export class Puppeteer {
   private _projectRoot: string;
-  _preferredRevision: string;
-  _isPuppeteerCore: boolean;
-  _changedProduct = false;
-  __productName: string;
+  private _isPuppeteerCore: boolean;
+  private _changedProduct = false;
+  private __productName: string;
   private _lazyLauncher: ProductLauncher;
+  /**
+   * @internal
+   */
+  _preferredRevision: string;
 
   /**
    * @internal
@@ -189,7 +195,7 @@ export class Puppeteer {
   }
 
   /**
-   * @returns The name of the browser that is under automation (`"chrome"` or `"firefox"`)
+   * The name of the browser that is under automation (`"chrome"` or `"firefox"`)
    *
    * @remarks
    * The product is set by the `PUPPETEER_PRODUCT` environment variable or the `product`