Improve the "Self closing tags" docs in guides/release/components/index.md

As part of https://github.com/ember-learn/guides-source/pull/2124 @kategengler noticed that the "self closing tags" part of the page is a bit odd and could do with updating. This is a tracking issue so we don't lose that TODO item 👍

Here is the discussion: https://github.com/ember-learn/guides-source/pull/2124/files?diff=split&w=1#r2214914990