Document 'Package Type'

[Faxmashine]

The following warning is shown on some pages of our documentation, i.e. https://vcc.docs.vrchat.com/guides/convert-unitypackage

We should add documentation about this warning. It's safe to ignore and prevents package content from showing up in Unity's search unless the user selects 'In Packages.'

[Faxmashine]

@orels1 pointed out where to find Unity's docs

https://docs.unity3d.com/2019.4/Documentation/Manual/upm-manifestPkg.html

'Reserved for internal use', how helpful 😅