website: add documentation for packer_http_addr extra variable

website/source/docs/provisioners/ansible.html.md

@@ -142,6 +142,13 @@ commonly useful Ansible variables:
     machine that the script is running on. This is useful if you want to run
     only certain parts of the playbook on systems built with certain builders.
 
+-   `packer_http_addr` If using a builder that provides an http server for file
+    transfer (such as hyperv, parallels, qemu, virtualbox, and vmware), this
+    will be set to the address. You can use this address in your provisioner to
+    download large files over http. This may be useful if you're experiencing
+    slower speeds using the default file provisioner. A file provisioner using
+    the `winrm` communicator may experience these types of difficulties.
+
 ## Debugging
 
 To debug underlying issues with Ansible, add `"-vvvv"` to `"extra_arguments"` to enable verbose logging.