workflows: creation rate limit (#20636)

cc @sidharthachatterjee

Author: elithrar

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

@@ -26,6 +26,7 @@ Many limits are inherited from those applied to Workers scripts and as documente
 | Maximum steps per Workflow [^5]           | 1024 [^1] | 1024 [^1] |
 | Maximum Workflow executions               | 100,000 per day [shared with Workers daily limit](/workers/platform/limits/#worker-limits) | Unlimited |
 | Concurrent Workflow instances (executions) per account   | 25 | 4500 [^1] |
+| Maximum Workflow instance creation rate | 100 per 10 seconds [^1][^6] | 100 per 10 seconds [^1][^6] | 
 | Maximum number of [queued instances](/workflows/observability/metrics-analytics/#event-types) | 10,000 [^1] | 100,000 [^1] |
 | Retention limit for completed Workflow state | 3 days | 30 days [^2] |
 | Maximum length of a Workflow ID [^4]         | 64 characters | 64 characters |
@@ -40,4 +41,6 @@ Many limits are inherited from those applied to Workers scripts and as documente
 
 [^5]: `step.sleep` do not count towards the max. steps limit
 
+[^6]: Workflows will return a HTTP 429 rate limited error if you exceed the rate of new Workflow instance creation.
+
 <Render file="limits_increase" product="workers" />