Add docs to a DCHECK in mojo bindings

Confusion about when to use Binding<> vs. BindingSet<> can cause an
attempt to bind to an interface that is already bound. There is an
existing DCHECK for this -- just add some docs to make that more clear.

Bug: 794581
Change-Id: I14194770190e90e80403c027a65e7820949331f8
Reviewed-on: https://chromium-review.googlesource.com/827903
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524260}

mojo/public/cpp/bindings/lib/binding_state.cc

@@ -87,7 +87,7 @@ void BindingStateBase::BindInternal(
     bool has_sync_methods,
     MessageReceiverWithResponderStatus* stub,
     uint32_t interface_version) {
-  DCHECK(!router_);
+  DCHECK(!is_bound()) << "Attempting to bind interface that is already bound.";
 
   auto sequenced_runner =
       GetTaskRunnerToUseFromUserProvidedTaskRunner(std::move(runner));