Clarify documentation for `inject.reference` and registry keys

[bcmills]

Currently the documentation for inject.reference is:

Return a reference to another registry key.

Unfortunately, at that point we haven't really defined what a “registry key” is: https://github.com/duolingo/minject/blob/master/docs/dependency_injection.md#storing-instantiated-classes suggests that it is a class, but generally the things we bind with @bind are instances rather than class objects.

It would be nice to clarify exactly what kind of thing the reference function returns and how that interacts with the other minject functions.