builder/vmware, builder/virtualbox: Document 'headless' option.

website/source/docs/builders/virtualbox.html.markdown

@@ -82,6 +82,13 @@ Optional:
   how to optimize the virtual hardware to work best with that operating
   system.
 
+* `headless` (bool) - Packer defaults to building VMware and VirtualBox
+  virtual machines by launching a GUI that shows the console of the
+  machine being built. When this value is set to true, the machine will
+  start without a console. For VMware machines, Packer will output VNC
+  connection information in case you need to connect to the console to
+  debug the build process.
+
 * `http_directory` (string) - Path to a directory to serve using an HTTP
   server. The files in this directory will be available over HTTP that will
   be requestable from the virtual machine. This is useful for hosting

website/source/docs/builders/vmware.html.markdown

@@ -77,6 +77,13 @@ Optional:
   OS type, VMware may perform some optimizations or virtual hardware changes
   to better support the operating system running in the virtual machine.
 
+* `headless` (bool) - Packer defaults to building VMware and VirtualBox
+  virtual machines by launching a GUI that shows the console of the
+  machine being built. When this value is set to true, the machine will
+  start without a console. For VMware machines, Packer will output VNC
+  connection information in case you need to connect to the console to
+  debug the build process.
+
 * `http_directory` (string) - Path to a directory to serve using an HTTP
   server. The files in this directory will be available over HTTP that will
   be requestable from the virtual machine. This is useful for hosting