Add sentence specifying what dtype elements must have.

Author: rgommers

spec/API_specification/dataframe_api/column_object.py

@@ -13,7 +13,9 @@ def from_sequence(cls, sequence: Sequence[object], dtype: dtype) -> Column:
         Parameters
         ----------
         sequence : Sequence[object]
-            Sequence of elements.
+            Sequence of elements. Each element must be of the specified
+            ``dtype``, the corresponding Python builtin scalar type, or
+            coercible to that type.
         dtype : str
             Dtype of result. Must be specified.