Followup to r59536: make "is_argument_optional" available to py, and use it in API doc generation.

Thanks Campbell for the much better name suggestion!

release/scripts/modules/rna_info.py

@@ -296,6 +296,8 @@ class InfoPropertyRNA:
         elif as_arg:
             if not self.is_required:
                 type_info.append("optional")
+            if self.is_argument_optional:
+                type_info.append("optional argument")
         else:  # readonly is only useful for self's, not args
             if self.is_readonly:
                 type_info.append("readonly")