[Workflow] Make sure all capabilities are documented (purge, etc)

[hhunter-ms]

What content needs to be created or modified?
Need more documentation about purge capability for workflow in the features/concepts doc. May also need to add other capabilities that haven't been documented.

Describe the solution you'd like
Section/paragraph, bullet list, cross linking

Where should the new material be placed?
Workflow features and concepts

The associated pull request from dapr/dapr, dapr/components-contrib, or other Dapr code repos
As mentioned in a comment here: #3969

[msfussell]

Specifically I would add the following

1. Before this section https://docs.dapr.io/developing-applications/building-blocks/workflow/workflow-features-concepts/#limitations add a title called "Purging" with text something like this
   "Workflow state can be purged from a state store for which purges all its history and removes all metadata related to a specific workflow instance. This is typically used for workflows that have run to a COMPLETED or TERMINATED state. Purging is also performed when a workflow moves to a CONTINUED_AS_NEW state. There are of course other times that you may choose to purge a workflow history and reuse a workflow instance with new inputs, for example with a FAILED workflow instance"

2. In the https://docs.dapr.io/developing-applications/building-blocks/workflow/workflow-architecture/#state-store-usage section add that "Workflow state can be purged from a state store for which purges all its history. Purging removes all metadata related to a specific workflow instance"

I would get @cgillum to review this text suggestion.