Fix Issue 13547 - "D is a fully garbage collected language"

[ntrel]

https://issues.dlang.org/show_bug.cgi?id=13547

Supercedes #666 - I squashed those commits together, then made the suggested edits in a separate commit. I also weakened some of the wording on downsides of GC in the 3rd commit, because:

• AIUI, the GC only collects when making an allocation (it doesn't interrupt).
• A D GC could do partial collections, preventing long pauses.
• Threads not managed by the runtime do not necessarily have to be paused when collecting garbage.