x86, doc: Revert "x86: Document rdmsr_safe restrictions"

This reverts commit ce37defc
"x86: Document rdmsr_safe restrictions", as these restrictions no longer apply.

Reported-by: Borislav Petkov <borislav.petkov@amd.com>
Link: http://lkml.kernel.org/r/20120419171609.GH3221@aftab.osrc.amd.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>