Document typed certificate credential use

[vito]

Referencing:

• #156
• #160
• #139

We should clearly document somewhere that these vars are typed vars, specifically type: certificate, and need individual fields specified. I'm not sure where exactly to put these docs yet, but I'll open this issue and keep referencing it as long as we keep getting these PRs. Hopefully we can figure out where to document this. 🤔