Update docs/source/model_library.rst

Co-authored-by: Ned Molter <[email protected]>

docs/source/model_library.rst

@@ -400,7 +400,8 @@ container unchanged for other steps.
 A survey of container usage was performed with a few key findings:
 
 - Many uses could be replaced by simpler containers (lists)
-- When loaded from an association, the container size never changed
+- When loaded from an association, the container size never changed;
+  that is, no use-cases required adding new models to associations within steps
 - The order of models was never changed
 - Needs various methods for stpipe
 - Several steps implemented different memory optimizations