Add a test that multiple nics work in google_compute_instance. (hashi…

…corp#289)

* Add a test that multiple nics works in google_compute_instance.

* Add note that multiple nics is in Beta.

docs/r/compute_instance.html.markdown

@@ -71,8 +71,7 @@ The following arguments are supported:
 * `zone` - (Required) The zone that the machine should be created in.
 
 * `network_interface` - (Required) Networks to attach to the instance. This can
-    be specified multiple times for multiple networks, but GCE is currently
-    limited to just 1. Structure is documented below.
+    be specified multiple times; multiple `network_interface` support is in Beta. Structure is documented below.
 
 - - -