docs: add Warns section to _get_affine docstring documenting the missing-tag warning

Signed-off-by: Raphael Malikian <rtmalikian@gmail.com>

Author: rtmalikian

monai/data/image_reader.py

@@ -735,6 +735,11 @@ def _get_affine(self, metadata: dict, lps_to_ras: bool = True):
             metadata: metadata with dict type.
             lps_to_ras: whether to convert the affine matrix from "LPS" to "RAS". Defaults to True.
 
+        Warns:
+            UserWarning: when ImageOrientationPatient (00200037) or ImagePositionPatient
+                (00200032) is missing from metadata. The affine matrix is set to identity,
+                which may be incorrect. Common with multiframe DICOM files.
+
         """
         affine: np.ndarray = np.eye(4)
         if not ("00200037" in metadata and "00200032" in metadata):