This kind of looks like a misuse of the external link, which I don’t think you should except to be allowed anytime soon.
I’d rather use footnotes for stuff like that, or the custom option discussed in the thread below.
Using footnotes you would enter text like:
Obsidian^[a hard, dark, glasslike volcanic rock] is the private and flexible writing app that adapts to the way you think.
Which if that was the only text renders as:
Using the tip of the other thread, depending on hovering or not, one could write something like:
<span data-tip="(a hard, dark, glasslike volcanic rock)">Obsidian</span> is the private and flexible writing app that adapts to the way you think.
And get the output:
or when hovered:
Hope this helps to some extent. 