i386: Add warning in Documentation that zero-page is not a stable ABI

Some people writing boot loaders seem to falsely belief the 32bit zero page is a
stable interface for out of tree code like the real mode boot protocol. Add a comment
clarifying that is not true.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>