Add API documentation for virtual external cameras

Since the 25Q2 release has now its own release branch, modify the
VirtualDeviceManager API documentation around the recently added
support for virtual external cameras under the external_virtual_cameras
flag.

Bug: 406957588
Test: N/A, doc only changes
Flag: EXEMPT, doc only changes
Change-Id: I97ca588d166cfc88812ca0ee7bf625c5f87f3746