Clarify supported ISO urls documentation in shared partials as not all builders support VHD

[nywilken]

This issue came up in a discussion with vSphere folks. The embedded partials contain information that is not true for the vSphere builder. We need to clarify the documentation to read that some builders may support additional media such as VHD or VHDX and encourage users to check the builder-specific documentation for usage instructions.

iso_url (string) - A URL to the ISO containing the installation image or virtual hard drive (VHD or VHDX) file to clone.

Optional:
iso_urls ([]string) - Multiple URLs for the ISO to download. Packer will try these in order. If anything goes wrong attempting to download or while downloading a single URL, it will move on to the next. All URLs must point to the same file (same checksum). By default this is empty and iso_url is used. Only one of iso_url or iso_urls can be specified.