edit: don't bother, I found it.
Maybe someone else might benefit from knowing where you got it. Saves them asking the same thing, see.
I installed the manual v2.26 using the package manager on my system.
Not that it helped me much because also that version did not have the description for pthreads.
Then I remembered that, when I wrote the software using pthreads, I used a book called
"The Linux programming interface" by Michael Kerrisk, as a reference...

So, I haven't found any "official"
online manual for POSIX threads. If somebody now where to find it, I would like to hear it.