- add documentation that describes, then proceeds to warn about

- add documentation that describes, then proceeds to warn about
the many caveats and confusing effects of, the popular approach
of using contains_eager() to alter the natural result of a
related collection.  I'm not a fan of this technique as it changes
the semantics of a relationship in such a way that the rest of the
ORM isn't aware of and it also can be undone very easily; hence the
section needs as much text for warnings as for describing the
technique itself.   fixes #3563