snailboat has begun writing footnotes for explaining notation. This is a good idea, but it adds clutter to answers.
Another way to { do | *be doing } it occured to me.
Clicking on "*be doing" it can take the user to some FAQ text, and the tooltip help explains it inline when you hover the mouse pointer over it.
The downside is the cumbersome markup:
<a href="http://example.com" title="the * notation means that this choice is ungrammatical">*be doing</a>
<a>
is allowed, and that thetitle
attribute is allowed on<a>
.[*be doing](http://example.com "the * notation means that this choice is ungrammatical")