Add more visible warnings to C-API functions that steal references of their arguments.

Cython/Includes/cpython/list.pxd

@@ -42,17 +42,19 @@ cdef extern from "Python.h":
 
     int PyList_SetItem(object list, Py_ssize_t index, object item) except -1
     # Set the item at index index in list to item. Return 0 on success
-    # or -1 on failure. Note: This function ``steals'' a reference to
-    # item and discards a reference to an item already in the list at
-    # the affected position.
+    # or -1 on failure.
+    #
+    # WARNING: This function _steals_ a reference to item and discards a
+    # reference to an item already in the list at the affected position.
 
     void PyList_SET_ITEM(object list, Py_ssize_t i, object o)
     # Macro form of PyList_SetItem() without error checking. This is
     # normally only used to fill in new lists where there is no
-    # previous content. Note: This function ``steals'' a reference to
-    # item, and, unlike PyList_SetItem(), does not discard a reference
-    # to any item that it being replaced; any reference in list at
-    # position i will be *leaked*.
+    # previous content.
+    #
+    # WARNING: This function _steals_ a reference to item, and, unlike
+    # PyList_SetItem(), does not discard a reference to any item that
+    # it being replaced; any reference in list at position i will be *leaked*.
 
     int PyList_Insert(object list, Py_ssize_t index, object item) except -1
     # Insert the item item into list list in front of index

Cython/Includes/cpython/module.pxd

@@ -191,9 +191,10 @@ cdef extern from "Python.h":
 
     int PyModule_AddObject(object module,  const char *name, object value) except -1
     # Add an object to module as name. This is a convenience function
-    # which can be used from the module's initialization
-    # function. This steals a reference to value. Return -1 on error,
-    # 0 on success.
+    # which can be used from the module's initialization function.
+    # Return -1 on error, 0 on success.
+    #
+    # WARNING: This _steals_ a reference to value.
 
     int PyModule_AddIntConstant(object module,  const char *name, long value) except -1
     # Add an integer constant to module as name. This convenience

Cython/Includes/cpython/tuple.pxd

@@ -47,13 +47,15 @@ cdef extern from "Python.h":
 
     int PyTuple_SetItem(object  p, Py_ssize_t pos, object  o) except -1
     # Insert a reference to object o at position pos of the tuple
-    # pointed to by p. Return 0 on success. Note: This function
-    # ``steals'' a reference to o.
+    # pointed to by p. Return 0 on success.
+    #
+    # WARNING: This function _steals_ a reference to o.
 
     void PyTuple_SET_ITEM(object  p, Py_ssize_t pos, object  o)
     # Like PyTuple_SetItem(), but does no error checking, and should
-    # only be used to fill in brand new tuples. Note: This function
-    # ``steals'' a reference to o.
+    # only be used to fill in brand new tuples.
+    #
+    # WARNING: This function _steals_ a reference to o.
 
     int _PyTuple_Resize(PyObject **p, Py_ssize_t newsize) except -1
     # Can be used to resize a tuple. newsize will be the new length of