Adjusted generalized type alias feature spec to indicate ongoing discussion

[eernstg]

The feature specification for constructs like typedef F<X> = List<X>; was added a few days ago, but it turns out that we need to discuss one element of the design ("is it allowed to use a type alias as a class?, e.g. class C extends F<int> {}, new F(), where F denotes a type alias").

This CL makes it explicit that that particular decision has not yet been made, and it indicates two likely outcomes of the discussion. This should be sufficient to put readers in a good position to understand what's going on.

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of all the commit author(s), set the cla label to yes (if enabled on your project), and then merge this pull request when appropriate.