Merge pull request #4020 from hhunter-ms/issue_4003

[Workflow] Add more documentation around purging

Author: hhunter-ms

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, including all its history. Each Dapr SDK exposes APIs for purging all metadata related to specific workflow instances.
+
 ## 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

@@ -63,6 +63,8 @@ You can use the following two techniques to write workflows that may need to sch
 
 1. **Use the _continue-as-new_ API**:  
     Each workflow SDK exposes a _continue-as-new_ API that workflows can invoke to restart themselves with a new input and history. The _continue-as-new_ API is especially ideal for implementing "eternal workflows", like monitoring agents, which would otherwise be implemented using a `while (true)`-like construct. Using _continue-as-new_ is a great way to keep the workflow history size small.
+   
+    > The _continue-as-new_ API truncates the existing history, replacing it with a new history.
 
 1. **Use child workflows**:  
     Each workflow SDK exposes an API for creating child workflows. A child workflow behaves like any other workflow, except that it's scheduled by a parent workflow. Child workflows have:
@@ -149,6 +151,12 @@ Workflows can also wait for multiple external event signals of the same name, in
 
 Learn more about [external system interaction.]({{< ref "workflow-patterns.md#external-system-interaction" >}})
 
+## 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 used for workflows that have run to a `COMPLETED`, `FAILED`, or `TERMINATED` state. 
+
+Learn more in [the workflow API reference guide]({{< ref workflow_api.md >}}).
+
 ## Limitations
 
 ### Workflow determinism and code restraints 

daprdocs/content/en/reference/api/workflow_api.md

@@ -174,6 +174,10 @@ Purge the workflow state from your state store with the workflow's instance ID.
 POST http://localhost:3500/v1.0-beta1/workflows/<workflowComponentName>/<instanceId>/purge
 ```
 
+{{% alert title="Note" color="primary" %}}
+Only `COMPLETED`, `FAILED`, or `TERMINATED` workflows can be purged.
+{{% /alert %}}
+
 ### URL parameters
 
 Parameter | Description