Beefed up description of what this does; new XXX.
diff --git a/Include/pyarena.h b/Include/pyarena.h
index 1636556..f5e9eb4 100644
--- a/Include/pyarena.h
+++ b/Include/pyarena.h
@@ -12,8 +12,16 @@
/* PyArena_New() and PyArena_Free() create a new arena and free it,
respectively. Once an arena has been created, it can be used
- to allocate memory. Once it is freed, all the memory it allocated
- is freed and none of its pointers are valid.
+ to allocate memory via PyArena_Malloc(). Pointers to PyObject can
+ also be registered with the arena via PyArena_AddPyObject(), and the
+ arena will ensure that the PyObjects stay alive at least until
+ PyArena_Free() is called. When an arena is freed, all the memory it
+ allocated is freed, the arena releases internal references to registered
+ PyObject*, and none of its pointers are valid.
+ XXX (tim) What does "none of its pointers are valid" mean? Does it
+ XXX mean that pointers previously obtained via PyArena_Malloc() are
+ XXX no longer valid? (That's clearly true, but not sure that's what
+ XXX the text is trying to say.)
PyArena_New() returns an arena pointer. On error, it
returns a negative number and sets an exception.