pathconf.2: Document the _PC_HAS_NAMEDATTR name

Commit 0f12c3cd0ddb adds a new pathconf(2) name called
_PC_HAS_NAMEDATTR.

This patch documents it in the man page.

This is a content change.

Reviewed by: ziaee (manpages)
Differential Revision: https://reviews.freebsd.org/D50141
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")