DOC: clarify behavior of 'cols' parameter in ApplyToCols and ApplyToFrame (#1670)

Author: kudos07

skrub/_apply_to_cols.py

@@ -204,6 +204,7 @@ class ApplyToCols(TransformerMixin, BaseEstimator):
     """Map a transformer to columns in a dataframe.
 
     A separate clone of the transformer is applied to each column separately.
+    All columns not listed in ``cols`` remain unmodified in the output.
     Moreover, if ``allow_reject`` is ``True`` and the transformers'
     ``fit_transform`` raises a ``RejectColumn`` exception for a particular
     column, that column is passed through unchanged. If ``allow_reject`` is
@@ -233,9 +234,10 @@ class ApplyToCols(TransformerMixin, BaseEstimator):
         the output.
 
     cols : str, sequence of str, or skrub selector, optional
-        The columns to attempt to transform. Columns outside of this selection
-        will be passed through unchanged, without attempting to call
-        ``fit_transform`` on them. The default is to attempt transforming all
+        The columns to attempt to transform. Only the selected columns will have
+        the transformer applied. Columns outside this selection are passed
+        through unchanged (``fit_transform`` is not called on them) and remain
+        unmodified in the output. The default is to attempt transforming all
         columns.
 
     allow_reject : bool, default=False

skrub/_apply_to_frame.py

@@ -14,6 +14,7 @@ class ApplyToFrame(TransformerMixin, BaseEstimator):
     A subset of the dataframe is selected and passed to the transformer (as a
     single input). This is different from ``ApplyToCols``, which fits a
     separate clone of the transformer to each selected column independently.
+    All columns not listed in ``cols`` remain unmodified in the output.
 
     .. note::
 
@@ -30,9 +31,10 @@ class ApplyToFrame(TransformerMixin, BaseEstimator):
         will appear unchanged in the output.
 
     cols : str, sequence of str, or skrub selector, optional
-        The columns to attempt to transform. Columns outside of this selection
-        will be passed through unchanged, without calling ``fit_transform`` on
-        them. The default is to transform all columns.
+        The columns to attempt to transform. Only the selected columns will have
+        the transformer applied. Columns outside of this selection are passed
+        through unchanged (``fit_transform`` is not called on them) and remain
+        unmodified in the output. The default is to transform all columns.
 
     keep_original : bool, default=False
         If ``True``, the original columns are preserved in the output. If the