nsswitch.conf(5): recommend placing cache after files

When cache precedes files, and nscd is configured to allow negative caching,
commands like "pw groupadd" can fail. The sequence of events looks like:

1. A command like pkg(8) looks up the group, and finds it absent.
2. pkg invokes pw(8) to add the group
3. pkg queries the group, but nscd says it doesn't exist, since it has a negative cache entry for that group.

See also: https://lists.freebsd.org/pipermail/freebsd-current/2012-January/031595.html

Reviewed by: bcr (manpages)
MFC after: 1 week
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D26184