pathconf.2: Document the _PC_HAS_NAMEDATTR name

[rmacklem]

Summary

D50140 adds a new pathconf(2) name called
_PC_HAS_NAMEDATTR.

This patch documents it in the man page.

[ziaee]

Are these Function Arguments? Can I alphabetize these?

[rmacklem]

In D50141#1143706, @ziaee wrote:

Are these Function Arguments? Can I alphabetize these?

I suspect they are ordered from when they are added, in
increasing numerical order.
Some are defined it POSIX and I think those should remain
first (since they are the most widely used), but that's just
my opinion.
(If you look in sys/unistd.h you can see which ones are POSIX.)