x86/uaccess: Document copy_from_user_nmi()

Document the functionality of copy_from_user_nmi() to avoid further
confusion. Fix the typo in the existing comment while at it.

Requested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201117202753.806376613@linutronix.de