Depending on a narrower scope is not an error

This [documentation page](https://github.com/google/guice/wiki/InjectingProviders#providers-for-mixing-scopes) states:

> It is an error to depend on an object in a _narrower_ scope

However, this is not enforced, so not truly an error. An exception should either be thrown when injecting an object with an object of a narrower scope, or the documentation should be revised to reflect that this will merely cause unintended results but is not an 'error'.