Update pricing.mdx

Author: elithrar

src/content/docs/workflows/reference/pricing.mdx

@@ -35,6 +35,10 @@ No. Workflows are priced based on the same compute (CPU time) and requests (invo
 
 Yes.
 
+### What is a Workflow invocation?
+
+A Workflow invocation is when you trigger a new Workflow instance: for example, via the [Workers API](/workflows/build/workers-api/), wrangler CLI, or REST API. Steps within a Workflow are not invocations.
+
 ### How do Workflows show up on my bill?
 
 Workflows are billed as Workers, and share the same CPU time and request SKUs.