The post upload doesn't seem to indicate what formatting / syntax is allowed within the description.

Is it the same as the help links I see here in the new forum post page?

https://e621.net/help/show/dtext

I want to be sure that the description is not pure text only.

Post descriptions appear to support dtext the same way as everything else does.

Even if it didn't, you could always just edit any descriptions you add.

Aside from a couple things (colors, anchors), descriptions support full DText (so even things like sections and tables work).