acl_is_trivial_np(3): minor fixes

• Correct the document title
• This function has an _np suffix
• acl_strip_np has a manpage, xref it
• RETURN VALUES should describe acl_is_trivial_np, not acl_get_tag_type

Sponsored by: Klara, Inc.

(cherry picked from commit 0e96337b11910b444763439a722c89983d660a09)