Docs: Note that DISALLOW_CONFIG_VPN stops VPNs from starting

VPNs don't start except a DPC's configured always-on VPN. Clarified version behaviors.
Staged at: go/dac-stage/reference/android/os/UserManager.html#DISALLOW_CONFIG_VPN

Test: make ds-docs and output staged UserManager page
Bug: 63582789
Change-Id: I83c0c6cc580b9d1d224a94e073f559f23cf4d672