Render icons based on quality of terms in Usage Dictionary

[daobrien]

NB: this is a low priority request, and is related to #161

The style guide uses <variablelistentry> elements for each of the listed terms. I'd like to introduce suitable icons to improve visibility of correct usage.

Copied from chat with @niklucas

<term role="true">air gap</term>
<term role="false">air wall</term>
<term role="caution">air vent</term>

For each of true, false, and caution I'd like to render either a green :heavy_check_mark: or red :x: or red :exclamation: to highlight the nature of the term. The text of the definition would then go on to properly explain how to use or not use each term.