gh-121533: Improve PyCell_[Get,Set] docs: mention the exceptions (#121534)

Co-authored-by: Victor Stinner <[email protected]>

Author: sobolevn

Doc/c-api/cell.rst

@@ -39,7 +39,8 @@ Cell objects are not likely to be useful elsewhere.
 
 .. c:function:: PyObject* PyCell_Get(PyObject *cell)
 
-   Return the contents of the cell *cell*.
+   Return the contents of the cell *cell*, which can be ``NULL``.
+   If *cell* is not a cell object, returns ``NULL`` with an exception set.
 
 
 .. c:function:: PyObject* PyCell_GET(PyObject *cell)
@@ -52,8 +53,10 @@ Cell objects are not likely to be useful elsewhere.
 
    Set the contents of the cell object *cell* to *value*.  This releases the
    reference to any current content of the cell. *value* may be ``NULL``.  *cell*
-   must be non-``NULL``; if it is not a cell object, ``-1`` will be returned.  On
-   success, ``0`` will be returned.
+   must be non-``NULL``.
+
+   On success, return ``0``.
+   If *cell* is not a cell object, set an exception and return ``-1``.
 
 
 .. c:function:: void PyCell_SET(PyObject *cell, PyObject *value)