chore: add comment explaining tuple conversion

Author: Tom McCarthy

aws_lambda_powertools/event_handler/api_gateway.py

@@ -688,6 +688,7 @@ def route(
         cache_control: Optional[str] = None,
     ):
         def register_route(func: Callable):
+            # Convert methods to tuple. It needs to be hashable as its part of the self._routes dict key
             methods = (method,) if isinstance(method, str) else tuple(method)
             self._routes[(rule, methods, cors, compress, cache_control)] = func