Document tricks for using a native debugger with nontrivial JS

[jdm]

1. call dump_js_stack(cx)
2. b math_sin and sprinkle Math.sin(0) whether you want a breakpoint

[jdm]

p ((servo_url::ServoUrl*)request->url_list.buf.inner.ptr.pointer.pointer)->__0.p.pointer->data.serialization when finding out the url for a request in a network thread.

[jdm]

frame variable cx.0 to obtain the actual JSContext pointer for call dump_js_stack(...)