Improve docs for raw.to_data_frame (mne-tools#13590)

Co-authored-by: Daniel McCloy <dan@mccloy.info>

Author: 1himan

mne/io/base.py

@@ -2429,6 +2429,10 @@ def to_data_frame(
         additional column "time" is added, unless ``index`` is not ``None``
         (in which case time values form the DataFrame's index).
 
+        .. attention:: By default, returned data values are scaled from
+            SI units to a unit more suitable for plotting or statistical modeling.
+            See the description of the ``scalings`` parameter for details.
+
         Parameters
         ----------
         %(picks_all)s

mne/utils/docs.py

@@ -3982,7 +3982,8 @@ def _reflow_param_docstring(docstring, has_first_line=True, width=75):
 scalings : dict | None
     Scaling factor applied to the channels picked. If ``None``, defaults to
     ``dict(eeg=1e6, mag=1e15, grad=1e13)`` — i.e., converts EEG to µV,
-    magnetometers to fT, and gradiometers to fT/cm.
+    magnetometers to fT, and gradiometers to fT/cm. See :term:`data channels`
+    and :term:`non-data channels` for full list of default scalings.
 """
 
 docdict["scalings_topomap"] = """