Document justification for dangerouslySetInnerHTML, fixes #2256

I put this into the "tips" section. Perhaps this should be in the "reference" section - let me know if you have a preference.
dangerouslySetInnerHTML is a prop name, not a parameter.

Also, saying we force people to look up docs isn't really right. We would have written them 18 months ago if that were the case. The truth is that we *really* want people to opt in.