Document the 1 that was in the definition, but not in the prose.

Author: Derick Rethans

source/includes/apiargs-dbcommand-aggregate-field.yaml

@@ -1,7 +1,7 @@
 arg_name: field
 description: |
   The name of the collection or view that acts as the input for the
-  aggregation pipeline.
+  aggregation pipeline. Use ``1`` for collection agnostic commands.
 interface: dbcommand
 name: aggregate
 operation: aggregate