Add a helpful warning msg about case sensitivity on isolated scope variable names

I just spent several hours banging my head because of an undefined value with isolating scope. I talked to another engineer who had the same problem and he just gave up after wasting 1/2 a day. I eventually guessed that it might be a case sensitivity issue. I just want to pay it forward and help people from giving up on this!!!!
It's not super clear what you're saying here.

Do you mean "variable name" or "attribute name"? case normalization rules for attributes are a bit confusing, but the rules are documented here https://docs.angularjs.org/guide/directive#matching-directives ---- attribute name for isolate scope variables can be either the scope property name (if none is specified), or the attribute name (the string following the `=`, `%` or `&`) --- both of these cases represent normalized attribute names.

I think we should write this in a clearer way, and provide a link to the rules of attribute normalization.