use a definition list instead of ad-hoc markup for definitions

At first, I was apprehensive about adding HTML to the spec doc, but then I noticed that the [basic concepts](https://github.com/php/php-langspec/blob/211703ae6a6e658bb0e911902f42923ac025b88b/spec/04-basic-concepts.md) page already has some `<pre>` HTML, and it really should up to the rendering code to decide how to display stuff like lists.