inet(3): clarify syntax accepted by inet_pton

[karels]

Summary

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.)

[pauamma_gundo.com]

English LGTM.

[karels]

Anyone from manpages want to approve this?