docs(batches): explain credential migration (#607)

Based on
https://sourcegraph.slack.com/archives/C012V43N8E7/p1724928676289499?thread_ts=1723741990.954679&cid=C012V43N8E7

For
https://linear.app/sourcegraph/issue/SRCH-946/how-do-we-handle-open-batch-changes-when-switching-authentication

Author: bahrmichael

docs/batch-changes/configuring-credentials.mdx

@@ -201,6 +201,14 @@ GitHub apps follow the same concepts as [personal and global access tokens](#typ
 - GitHub apps can only be used with GitHub code hosts.
 - The forking mechanism (`fork:true`) is only supported if the GitHub app has access to all repositories of the installation, and if the origin repository and the fork are in the same organization that the GitHub app is installed to ([GitHub Docs](https://docs.github.com/en/rest/repos/forks?apiVersion=2022-11-28#create-a-fork)).
 
+### Migrating from PATs to GitHub Apps
+
+You can migrate your credentials from PATs to GitHub Apps by deleting the PAT credential and creating a GitHub app credential.
+
+Batch Changes will look at the available credentials, and pick one that matches the targeted namespace (e.g. organization).
+
+You can continue to use existing batch changes without modifications.
+
 ### Adding a GitHub app
 
 Adding a GitHub app is done through the Batch Changes section of your user settings: