inet(3): clarify syntax accepted by inet_pton

The section INTERNET ADDRESSES describes the acceptance of dotted
values with varying number of parts in multiple bases. This applies
to inet_aton and inet_addr, but not to inet_pton. Clarify this
section by listing the functions to which this applies. Move the
description of what inet_pton accepts into this section from STANDARDS,
where it is easily missed. Rename the section to clarify that it
applies only to IPv4. (inet_pton also works with IPv6.)

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43537