Improve ReactDOM's uppercase HTML warning

[nickserv]

Do you want to request a feature or report a bug?
Improvement request/discussion

What is the current behavior?
Rendering <fooBar/> in ReactDOM:

fooBar is using uppercase HTML. Always use lowercase HTML tags in React.

What is the expected behavior?
The warning message can be improved to make it more obvious how to fix the issue for different use cases. For example, it's not obvious from this message that custom components should add dashes or that the first letter should be capitalized if it's actually a React component. Here's my idea, but I'm open to suggestions:

Warning: <foorBar /> looks like an HTML or web component element because it starts with a lowercase letter, but it has uppercase letters instead of dashes. Use PascalCase for React components, or lowercase letters and dashes for HTML.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
master: eb6e752

[gaearon]

Sure, send a PR?

[wherestheguac]

I jumped on this one since it was quick, PR here: #12533

[gaearon]

@nicolevy fixed this in #12533