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

[vangyzen]

Summary

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.

[kib]

share/man/man3/pthread_create.3

108

I do not like a reference to thr_new(2) in the pthrread_create(3), for the basic information about the function behaviour. I.e., I would be fine with .Xr thr_new as a note about implementation. But I think that it is wrong to refer to the implementation man page when describing behavior of the standard-mandated function. This removes the layer of implementation from the reader perception.

Please, list the missed errno values there explicitely.

[vangyzen]

List errno values directly in pthread_create(3) instead of referring
to the implementation's man pages.

[vangyzen]

share/man/man3/pthread_create.3

108

This makes sense.