docs: improve PR template explanation for SDK branch override

Author: bchampp

.github/pull_request_template.md

@@ -12,7 +12,8 @@ Brief description of the changes in this PR.
 - [ ] Added/updated tests for new functionality
 
 ## Dependencies
-If this PR requires changes from the Python Language SDK, specify the branch here:
+If this PR requires testing against a specific branch of the Python Language SDK (e.g., for unreleased changes), uncomment and specify the branch below. Otherwise, leave commented to use the main branch.
+
 <!-- PYTHON_LANGUAGE_SDK_BRANCH: branch-name -->
 
 ## Checklist