Clarify Dapr Workflow concurrency behavior

**Describe the issue**
Based on our testing, the `globalMaxConcurrentWorkflowInvocations` setting does not limit the number of active workflow instances. Instead, it appears to limit the number of concurrent workflow operations processed by the Workflow Scheduler Actor.

This behavior makes sense, but it was not immediately clear from the current documentation.

**URL of the docs**
https://docs.dapr.io/developing-applications/building-blocks/workflow/workflow-concurrency/

**Expected content**
The documentation could clarify that globalMaxConcurrentWorkflowInvocations does not limit the number of active workflow instances. Based on our testing, it limits only the number of concurrent workflow operations processed by the Workflow Scheduler Actor.

A short note describing this behavior and clarifying that the setting is not intended for workflow instance throttling or back-pressure purposes would help avoid misunderstandings.

Open the workflow concurrency documentation at the linked URL and review the section describing globalMaxConcurrentWorkflowInvocations. Add a short clarification that it limits concurrent workflow operations processed by the Workflow Scheduler Actor, not active workflow instances or workflow back-pressure; done when the distinction is explicit on the page.

Written by the indexing model from the issue text.