x86/vsyscall: Fix documentation to reflect the default mode

x86/vsyscall: Fix documentation to reflect the default mode

The default vsyscall mode has been updated from emulate to xonly for a
while. Update the kernel-parameters doc to reflect that.

Fixes: 625b7b7f79c6 ("x86/vsyscall: Change the default vsyscall mode to xonly")
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230111193211.1987047-1-sohil.mehta@intel.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>