Adapt Asset_kind docstring to be more precise

Previously, the docstring of `Asset_kind.Not_applicable` did not
reflect the newly added `Asset_kind.Role` attribute.
With this change, we make it more precise.

aas_core_meta/v3_1.py

@@ -2345,7 +2345,7 @@ class Asset_kind(Enum):
 
     Not_applicable = "NotApplicable"
     """
-    Neither a type asset nor an instance asset
+    Neither a type asset nor an instance asset nor a role asset
     """