The installables syntax is not documented in any of the man pages or
docbook pages for any of those individual commands. And while these
commands really should at least peripherally individually document how
installables work, in the meantime we can at least direct people to the
right place.
This commit also clarifies the unexpected fact that `nix profile remove`
and `nix profile upgrade` do *not* take installables.
Change-Id: I3b1453cb197a613bbab639c66a466365c3592c6d
194ba8a02f