Require a verbose changelog entry for all enhancement bugs

[wilzbach]

tl;dr: we should start treating enhancement "bugs" as new features and promote them as such to the users. They won't open the bug link, but will see a full text changelog notice. Motivation: https://github.com/dlang/dmd/pull/6880#issuecomment-362796817

Looking at the current changelog from https://dlang.org/changelog/pending.html

They all should have received a proper changelog entry. It can be a short one. Most of them even do, but the following ones were missed:

• Implement contracts without implementation.
• Cannot initialise contextless delegate at compile time
• Parameter type mismatch error message are not very helpful
• datetime toString functions should accept sink
• std.format.formattedRead should work with rvalues

and of course some "enhancements" were actually bugs

[CyberShadow]

Another instance: https://github.com/dlang/dmd/pull/12592