x86/realmode: Add comment for Global bit usage in trampoline_pgd

Document the fact that using the trampoline_pgd will result in the
creation of global TLB entries in the user range of the address
space.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211202153226.22946-2-joro@8bytes.org