Additions to proposal process to document feature/syntax use and error/warning scenarios

[jawbroken]

From a similar previous thread:

@jawbroken

I think brevity is a virtue, so I'm not sure it needs to be a standard component. It's always easy to see the benefit of adding new sections to the proposal template (encourages thought about more aspects of the design) but there are downsides too (takes up more time for both proposal writers and reviewers). Perhaps there could be some conditional sections, though (e.g. for proposals that change or add syntax, versus things like standard library additions).

I think this applies here too, where this might make sense for syntax changes but isn't generally necessary for additions to the standard library and similar.