Clarify attach_generator_files() doc strings (#287)

Clarify that invoking attach_generator_files() overwrites
previously attached files.

[ committed by @mellis13 ]
[ reviewed by @MattToast ]

smartsim/entity/model.py

@@ -109,7 +109,9 @@ def attach_generator_files(self, to_copy=None, to_symlink=None, to_configure=Non
         """Attach files to an entity for generation
 
         Attach files needed for the entity that, upon generation,
-        will be located in the path of the entity.
+        will be located in the path of the entity.  Invoking this method
+        after files have already been attached will overwrite
+        the previous list of entity files.
 
         During generation, files "to_copy" are copied into
         the path of the entity, and files "to_symlink" are