docs(batches): exclude forking public repos from supported github app workflows (#599)

See SRCH-983

I didn't verify if this works with PATs.

Author: bahrmichael

docs/batch-changes/configuring-credentials.mdx

@@ -194,6 +194,9 @@ When Sourcegraph is configured to [clone repositories using SSH via the `gitURLT
 
 EXPERIMENTAL: Using GitHub Apps to authenticate Batch Changes is currently experimental, and there may still be some rough edges.
 
+Features currently not supported are
+- running a batch change with `fork: true` that targets a public repository on github.com
+
 This is only available for GitHub code hosts.
 
 GitHub apps follow the same concepts as [personal and global access tokens](#types-of-access-tokens-used-by-batch-changes).