Add comment about what _PyObject_VirtualAlloc does.

Author: markshannon

Include/internal/pycore_pymem.h

@@ -94,7 +94,8 @@ struct _PyTraceMalloc_Config {
 
 PyAPI_DATA(struct _PyTraceMalloc_Config) _Py_tracemalloc_config;
 
-
+/* Allocate memory directly from the O/S virtual memory system,
+ * where supported. Otherwise fallback on malloc */
 void *_PyObject_VirtualAlloc(size_t size);
 void _PyObject_VirtualFree(void *, size_t size);