pthread_attr_get_np(3): Revamp

Fix and more thoroughly describe the attributes object lifecycle.

Also, correct the text about which attributes' values are reported.

Add the ENOMEM error.

While here, rephrase unclear passages, add references and fix the
example's style.

Reviewed by: kib, emaste
Approved by: emaste (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43330