More explicit warning docs for custom code + environments

Made the issues with using environments + custom ruby code (e.g. functions, facts, types, providers, etc) more explicit.

Currently, doing so is basically unsupportable/unworkable, and the existing docs made it sound like there are only occasionally issues.

We have frequent support tickets about this problem, and so I felt that making this into more of a [here be dragons](http://en.wikipedia.org/wiki/Here_be_dragons) type caveat was wise. The functionality basically doesn't work.