Show warnings when fixing names via StateFunc?

This is related to https://github.com/hashicorp/terraform/pull/4020 but also many other PRs which have been mimicking the AWS API behaviour in similar ways.

I understand the convenience for the user having existing resources and I understand that is the motivation for @stack72 and others to solve these issues that way. However I think that Terraform shouldn't be so silent about these fixes.

I think we should be more transparent and at least show warnings if the converted names differ from the original ones.

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I'd personally like to use warnings in a way that it would allow us to turn on strict validation in the future (after a few versions, when we're sure enough users have been warned), but I understand the convenience for existing users has probably priority and that others may not like this (correct me if I'm wrong).

Start by reviewing the issue discussion and the referenced Terraform pull request 4020, then trace how StateFunc name conversions currently behave in the SDK. Determine where warning behavior would belong and what warning-versus-strict-validation requirements need agreement. Done means the scope and expected behavior are documented well enough to implement and test.

Written by the indexing model from the issue text.