add purge to workflow docs

Signed-off-by: Hannah Hunter <hannahhunter@microsoft.com>

daprdocs/content/en/developing-applications/building-blocks/workflow/workflow-architecture.md

@@ -145,6 +145,8 @@ Different state store implementations may implicitly put restrictions on the typ
 
 Similarly, if a state store imposes restrictions on the size of a batch transaction, that may limit the number of parallel actions that can be scheduled by a workflow.
 
+Workflow state can be purged from a state store, purging all its history. Purging removes all metadata related to a specific workflow instance.
+
 ## Workflow scalability
 
 Because Dapr Workflows are internally implemented using actors, Dapr Workflows have the same scalability characteristics as actors. The placement service:

daprdocs/content/en/developing-applications/building-blocks/workflow/workflow-features-concepts.md

@@ -151,6 +151,15 @@ Learn more about [external system interaction.]({{< ref "workflow-patterns.md#ex
 
 ## Limitations
 
+### Purging
+
+Workflow state can be purged from a state store, purging all its history and removing all metadata related to a specific workflow instance. The purge capability is typically used for workflows that have run to a `COMPLETED` or `TERMINATED` state. 
+
+There are other times you may choose to purge a workflow history and reuse a workflow instance with new inputs, like: 
+
+- `CONTINUED_AS_NEW` state
+- A `FAILED` workflow instance
+
 ### Workflow determinism and code restraints 
 
 To take advantage of the workflow replay technique, your workflow code needs to be deterministic. For your workflow code to be deterministic, you may need to work around some limitations.