Merge pull request #3511 from edx/max/docs

Update docs on run_migrations.yml

playbooks/continuous_delivery/run_migrations.yml

@@ -13,11 +13,13 @@
 #   - APPLICATION_NAME                - The name of the application that we are migrating.
 #   - APPLICATION_USER                - user which is meant to run the application
 #   - ARTIFACT_PATH                   - the path where the migration artifacts should be copied after completion
+#   - DB_MIGRATION_USER               - the database username
+#   - DB_MIGRATION_PASS               - the database password
+#
+# Other variables:
 #   - HIPCHAT_TOKEN                   - API token to send messages to hipchat
 #   - HIPCHAT_ROOM                    - ID or name of the room to send the notification
 #   - HIPCHAT_URL                     - URL of the hipchat API  (defaults to v1 of the api)
-#
-# Other variables:
 #   - migration_plan                  - the filename where the unapplied migration YAML output is stored
 #   - migration_result                - the filename where the migration output is saved
 #   - SUB_APPLICATION_NAME            - used for migrations in edxapp {lms|cms}, must be specified