x86, 64-bit: update address space documentation

Impact: documentation update

Commit a6523748
(paravirt/x86, 64-bit: move __PAGE_OFFSET to leave a space for hypervisor)
changed address space without changing the documentation.

Change it according to the code change -- direct mapping start:
ffff810000000000 => ffff880000000000 which gives 57 TiB, something
between 45 and 46 bits.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>