Minor edits for all RFCs

[adpaco-aws]

Description of changes:

I noticed that @celinval 's RFCs contain an horizontal separator (e.g., this one) and I liked how it looks. So I'm proposing we add it to our template and already-merged RFCs.

In addition, this PR includes other minor fixes (missing RFC identifiers, status, etc.).

Call-outs:

When talking about the status, I'm not sure if cover-statement and should-panic-attr should be marked as "Stable", since they currently don't require any unstable-related option to enable them, but are relatively new to Kani (that's why I marked them as "Unstable").

Testing:

• How is this change tested? N/A

• Is this a refactor change? N/A

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.