Document library types in the intro(3) manual page

Add a paragraph about library types to the intro(3)
manual page. Document library types, locations
and versioning.

Reviewed by: emaste, jilles, mhorne, pauamma_gundo.com
Obtained from: OpenBSD (partial)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D36594