manual: clarify that scanf %n supports type modifiers

My initial reading of the %n documentation was that it didn't support
type conversions, because it only mentioned int*.

Corresponding man-pages patch:
https://lore.kernel.org/linux-man/20210328215509.31666-1-hi@alyssa.is/



Reviewed-by: Arjun Shankar <arjun@redhat.com>