> As far as I can see (looking at Python/random.c and configure.ac), the Solaris version should also use GRND_NONBLOCK:

Oh, you're right: I didn't notice that GRND_NONBLOCK was also used on Solaris. The change is not deliberate, but it is good to do that :-)

Solaris getrandom() is documented to fail with EAGAIN if "No entropy is available and GRND_NONBLOCK is set."
https://docs.oracle.com/cd/E53394_01/html/E54765/getrandom-2.html

The question is more if reading from /dev/urandom block in this case.

If we don't know, I would prefer to keep the "On Linux" prefix in the doc, and don't say anything about Solaris.

I'm able to check the behaviour of Solaris.

You should contact the developers who get access to Solaris, you can meet them in the previous random issues specific to Solaris: issue #25003 and issue #26735.