gh-145860: Document steal semantics in header declarations

Add comments to _PyInterpolation_Build and _PyTemplate_Build
declarations in pycore_interpolation.h and pycore_template.h.

Author: aleksandr.tseluyko

Include/internal/pycore_interpolation.h

@@ -13,6 +13,7 @@ extern PyTypeObject _PyInterpolation_Type;
 
 #define _PyInterpolation_CheckExact(op) Py_IS_TYPE((op), &_PyInterpolation_Type)
 
+// Steals references to value, str, and format_spec (even on failure).
 PyAPI_FUNC(PyObject *) _PyInterpolation_Build(PyObject *value, PyObject *str,
                                               int conversion, PyObject *format_spec);
 

Include/internal/pycore_template.h

@@ -17,6 +17,7 @@ extern PyTypeObject _PyTemplateIter_Type;
 
 extern PyObject *_PyTemplate_Concat(PyObject *self, PyObject *other);
 
+// Steals references to strings and interpolations (even on failure).
 PyAPI_FUNC(PyObject *) _PyTemplate_Build(PyObject *strings, PyObject *interpolations);
 
 #ifdef __cplusplus