Update 3.0.md

I have updated the text to try and address concerns raised in the TDC call.

- Clarified that compatibility with CommonMark is only relevant for tooling that renders rich text.  
- Clarified that tooling is not limited to only supporting CommonMark .27 features. Other newer features can be supported
- Clarified that tooling should only ignore and not fail when features with security concerns are used.