Improve default allocator wording

adoc/chapters/programming_interface.adoc

@@ -4918,8 +4918,10 @@ STL-based libraries (e.g, Intel's TBB provides an allocator).
 ==== Default allocators
 
 A default allocator is always defined by the implementation.
-For allocations greater than size zero, when successful it is guaranteed to
-return non-[code]#nullptr# and new memory positions every call.
+For successful allocations of size greater than zero, the default allocator
+must return a pointer to a contiguous, exclusive region of memory of at least
+the requested size.
+For unsuccessful allocations, the default allocator must return [code]#nullptr#.
 The default allocator for const buffers will remove the const-ness of the type
 (therefore, the default allocator for a buffer of type [code]#const int# will be
 an [code]#Allocator<int>)#.