pathconf.2: Document the _PC_HAS_HIDDENSYSTEM name

Commit afd5bc630930 added a new pathconf name
_PC_HAS_HIDDENSYSTEM.

This patch documents this new name.

This is a content change.

Reviewed by: kib, ziaee (manpages)
Differential Revision: https://reviews.freebsd.org/D51175
Fixes: afd5bc630930 ("pathconf: Add a new variable for hidden/system")