docs: rewrite spark DataFrame intro for users

Replace API-speak ("Import the submodule", "Returned values are Expr
instances that compose") with a concrete description of where users can
actually drop these calls.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/user-guide/common-operations/spark-functions.rst

@@ -45,9 +45,9 @@ implementation to call by which module you import from.
 DataFrame API
 -------------
 
-Import the submodule and call functions directly. Returned values are
-:py:class:`~datafusion.expr.Expr` instances that compose with the rest of
-the DataFrame API.
+Import ``spark`` and use it like any other functions module. The Spark
+functions can go anywhere you'd put a DataFusion expression — inside
+``select``, ``filter``, ``with_column``, ``aggregate``, and so on.
 
 .. code-block:: python