pathconf.2: Document the new _PC_CASE_INSENSITIVE name

Commit d6864221d8 added support for the _PC_CASE_INSENSITIVE
name for pathconf, to indicate if case insensitive lookups
are configured.

This patch documents this new name and also updates
the description for _PC_HAS_HIDDENSYSTEM to also
include UF_ARCHIVE.

This is a content change.

(cherry picked from commit b473bb274cef37d45940cad4624b5d3b7fc96e01)