Clarify ability to ask CCA to use different base branches from Copilot Chat (#56288)

Co-authored-by: hubwriter <hubwriter@github.com>

Author: nhu-do

content/copilot/concepts/about-copilot-coding-agent.md

@@ -102,7 +102,6 @@ Users can include hidden messages in issues assigned to {% data variables.produc
 * **{% data variables.product.prodname_copilot_short %} can only access context in the same repository as the assigned issue**. By default, an integration with the {% data variables.product.prodname_copilot_short %} MCP server provides {% data variables.product.prodname_copilot_short %} access to one repository at a time. You can, however, configure broader access. See [AUTOTITLE](/copilot/using-github-copilot/coding-agent/extending-copilot-coding-agent-with-mcp).
 * **{% data variables.product.prodname_copilot_short %} can only open one pull request at a time**. {% data variables.product.prodname_copilot_short %} will open exactly one pull request to address each task it is assigned.
 * **{% data variables.product.prodname_copilot_short %} cannot work on an existing pull request that it didn't create**. If you would like {% data variables.product.prodname_copilot_short %} to provide feedback on an existing pull request, you can add it as a reviewer. See [AUTOTITLE](/copilot/using-github-copilot/code-review/using-copilot-code-review).
-* **{% data variables.product.prodname_copilot_short %} will always start its changes from the repository's default branch**. {% data variables.product.prodname_copilot_short %} cannot branch off from any other branch—for example, a feature branch or a release branch.
 
 ### Limitations in Copilot's compatibility with other features
 

content/copilot/how-tos/agents/copilot-coding-agent/asking-copilot-to-create-a-pull-request.md

@@ -36,7 +36,7 @@ You can ask {% data variables.product.prodname_copilot_short %} to work on a tas
     For example, `Implement a user friendly message for common errors.`
 
   ![Screenshot of asking Copilot to create a pull request from the Agents page.](/assets/images/help/copilot/coding-agent/agents-page-input.png)
-  
+
 1. Click the **Send now** button or press <kbd>Return</kbd>.
 
     {% data variables.product.prodname_copilot_short %} will start a new session, which will appear in the list below the prompt box. {% data variables.product.prodname_copilot_short %} will work on the task and push changes to its pull request, then add you as a reviewer when it has finished, triggering a notification.
@@ -54,6 +54,10 @@ You can ask {% data variables.product.prodname_copilot_short %} to work on a tas
    > [!IMPORTANT]
    > {% data reusables.copilot.coding-agent.use-chat-participant-in-vsc %}
 
+   > [!TIP]
+   > * To help {% data variables.product.prodname_copilot_short %}, you can select the relevant line(s) of code before submitting your prompt.
+   > * From {% data variables.copilot.copilot_chat_short %}, you can ask {% data variables.product.prodname_copilot_short %} to open a pull request using a specific branch as the base branch by including it in your prompt.
+
 1. Submit your prompt.
 
    {% data variables.product.prodname_copilot_short %} asks you to confirm that you want to use the coding agent to create a pull request.