lguest: document 32-bit and PAE requirements

Robert noted that we don't actually document that lguest is 32-bit only,
nor that PAE must be off (CONFIG_PAE is now prompted for if HIGHMEM is
set to "off).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: lguest@ozlabs.org
Cc: "Robert P. J. Day" <rpjday@crashcourse.ca>