nsswitch.conf.5: Add list of files being read when "files" service is used

nsswitch.conf.5: Add list of files being read when "files" service is used

This is not mentioned anywhere. Users can assume that the file
being read is something like /etc/$DATABASE, but that's not
always the case. It's better to explicitly specify which
file is read for each respective database.  The list of
files was acquired from glibc source code.

Signed-off-by: Nikola Forró <nforro@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>