Add default join parameter values docs

src/concepts/metric_hub.md

@@ -88,8 +88,8 @@ Data sources can be joined with other data sources:
 # Join the `baseline` data source with the `metrics` data source.
 # Definitions for both data sources must exist.
 [data_sources.baseline.joins.metrics]
-relationship = "many_to_many"  # this determines the type of JOIN used; options: many_to_many, one_to_one, one_to_many, many_to_one
-on_expression = """  # SQL expression specifying the JOIN condition
+relationship = "many_to_many"  # this determines the type of JOIN used; options: many_to_many, one_to_one, one_to_many, many_to_one; default: many_to_many
+on_expression = """  # SQL expression specifying the JOIN condition; default join is on client_id_column and submission_date_columns
   baseline.client_id = metrics.client_id AND
   baseline.submission_date = metrics.submission_date
 """
@@ -100,7 +100,11 @@ Wildcard character can be used to apply joins to multiple data sources:
 ```toml
 # Apply join to all data sources prefixed with user_
 [data_sources.user_'*'.joins.metrics]
-# defaults are many_to_many relationship and joins on the client_id_column and submission_date_column
+# [default] relationship = many_to_many
+# [default] on_expression = """  # SQL expression specifying the JOIN condition; default join is on client_id_column and submission_date_columns
+#  baseline.{client_id_column} = metrics.{client_id_column} AND
+#  baseline.{submission_date_column} = metrics.{submission_date_column}
+# """
 ```
 
 ### `[metrics]` Section