local-exec: the OS doesn't need to be ready

It is not obvious that the resource being created doesn't mean that the OS and system services such as sshd are ready (contrary to `remote-exec`). It is better to make that explicit and same developers like me some headache :-)

website/source/docs/provisioners/local-exec.html.markdown

@@ -13,6 +13,9 @@ is created. This invokes a process on the machine running Terraform, not on
 the resource. See the `remote-exec` [provisioner](/docs/provisioners/remote-exec.html)
 to run commands on the resource.
 
+Beware that even though the resource is fully created when the provisioner is run,
+it doesn't need to have finished booting or starting system services.
+
 ## Example usage
 
 ```