MINOR: [Python][Documentation] Fix docstring of preserve_index in Schema.from_pandas (#37632)

### Rationale for this change

The documentation of the `preserve_index` keyword does not match its default behavior. This PR updates it to match the behavior also described in `Table.from_pandas`.

### What changes are included in this PR?

Updated documentation

### Are these changes tested?

No, documentation only

### Are there any user-facing changes?

Documentation only

Lead-authored-by: Hendrik Makait <[email protected]>
Co-authored-by: Alenka Frim <[email protected]>
Signed-off-by: AlenkaF <[email protected]>

Author: hendrikmakait

python/pyarrow/types.pxi

@@ -3116,12 +3116,12 @@ cdef class Schema(_Weakrefable):
         Parameters
         ----------
         df : pandas.DataFrame
-        preserve_index : bool, default True
-            Whether to store the index as an additional column (or columns, for
-            MultiIndex) in the resulting `Table`.
-            The default of None will store the index as a column, except for
-            RangeIndex which is stored as metadata only. Use
-            ``preserve_index=True`` to force it to be stored as a column.
+
+        preserve_index : bool, optional
+            Whether to store the index as an additional field in the resulting
+            ``Schema``. The default of None will store the index as a field,
+            except for RangeIndex which is stored as metadata only. Use
+            ``preserve_index=True`` to force it to be stored as a field.
 
         Returns
         -------