chore: add clarifying comment on jinja2.Environment import

Author: kiendang

graphql_server/render_graphiql.py

@@ -4,6 +4,8 @@
 import re
 from typing import Any, Dict, Optional, Tuple
 
+# This Environment import is only for type checking purpose,
+# and only relevant if rendering GraphiQL with Jinja
 try:
     from jinja2 import Environment
 except ImportError: