DOC: Clarify dtype of returned object from window ops (pandas-dev#43030)

Author: mroeschke

pandas/core/window/doc.py

@@ -16,7 +16,7 @@ def create_section_header(header: str) -> str:
 template_returns = dedent(
     """
     Series or DataFrame
-        Return type is the same as the original object.\n
+        Return type is the same as the original object with ``np.float64`` dtype.\n
     """
 ).replace("\n", "", 1)