docs: Document start_info changes in Xen 4.2.

The  25833:bb85bbccb1c9. "x86/32-on-64: adjust Dom0 initial page table layout"
fixes a bug in the reported value of pt_base versus where the page tables
actually start. This documents this in the start of the world header note.

This clarifies the implied understanding that the page table space is
pointed by pt_base. As in it is ".. implied that the range of page-tables
is the range [pt_base, pt_base + nr_pt_frames), whereas that that range
here indeed is [pt_base - 2, pt_base -2 + nr_pt_frames)" (Jan Beulich).

Also make it crystal clear that pt_base == %cr3.

Acked-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

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