Improve errno documentation in pthread_create(3) and thr_new(2)

Add some missing errno values to thr_new(2) and pthread_create(3).
In particular, EDEADLK was not documented in the latter.
While I'm here, improve some English and cross-references.

Reviewed by: kib
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D6663