Clarify BaseDynamicTable doc

Author: rly

common/table.yaml

@@ -95,7 +95,8 @@ groups:
 
 - data_type_def: BaseDynamicTable
   data_type_inc: Container
-  doc: A group containing multiple datasets that are aligned on the first dimension
+  doc: An abstract base type for a column-based table that can contain user-defined columns
+    This group contains multiple datasets that are aligned on the first dimension
     (Currently, this requirement is left up to APIs to check and enforce). These datasets
     represent different columns in the table. Apart from a column that contains unique
     identifiers for each row, there are no other required datasets. Users are free to add