improve docstring for load_from_external()

src/fixml/modules/template.py

@@ -39,6 +39,11 @@ def load_from_external(cls, ext_template_path: Union[str, Path],
         validate_template : str, optional
             If provided, the external template source will be compared with the
             referred internal template to confirm all variables are present.
+
+        Returns
+        -------
+        jinja2.Template
+            Source file loaded as a Jinja2 Template.
         """
         with open(ext_template_path, "r") as f:
             source = f.read()