Update DisplayDevice doc to include VirtualDisplay

The DisplayDevice doc explicity say that it represent a physical
display, which is not true anymore since it can also represent a virtual
display
(com.android.server.display.VirtualDisplayAdapter#createVirtualDisplayLocked)

Test: N/A
Bug: N/A
Change-Id: I010622ccbbc637d364f932082a8f96afbee8f6de