Is there a single word that means "to serve as documentation?"

(Documentation as in "this is how X works," not as in "here's my ID.")

Never mind. I reckon "documentary" can be an adjective as well as a noun.

But you asked for a verb: "to serve as..."

To document?

Documentable- documentables?
link

I don't think that any of the suggestion so far quite fit. I would go with documentation and drop the serves as. Example: "Here is how X works" yields "This is the documentation for X".

1. X has been documented.
2. X is documentable; now document it!
3. There is documentary evidence that X exists.

My two cents, adjusted for inflation.

I explained to a customer that a model serves two functions:
1) Declarative / Documentary and 2) Interrogative / Investigative.

In the first case, one can ask questions like "How does this widget get from point A to point B?" or "What exactly is that fellow's job?" In the second, "What are the likely consequences of allocating resource X in this other way?" or "What happens if I route that material in this other direction?"

Even technically-minded people appreciate concise, lucid explanations. Your checks are in the mail.

tx,
tff