Nuget docs update

[AnthonySteele]

• Updated the buildpackage docs This is not just for prereleases any more
• Removed the -prerelease switch from this script
• it was not flexible enough, we need multiple prereleases with different names
• the -v switch achieves this by allowing you to specify any version number/name
• better docs now explain how to use -v to generate a named build, potentially prerelease