doc: document make doconly

Documented `make doconly` as a way to read documentation in the browser.

BUILDING.md

@@ -206,15 +206,14 @@ To read the documentation:
 $ man doc/node.1
 ```
 
-If you prefer to read the documentation in the browser and have `python` 
-installed, you can run:
+If you prefer to read the documentation in the browser,
+run the following once `make doc` is done:
 
 ```console
-$ cd out/doc/api/
-$ python -m SimpleHTTPServer
+$ make doconly
 ```
 
-and navigate to `http://localhost:8000` in your browser.
+This will open up a browser tab/window with the documentation.
 
 To test if Node.js was built correctly: