Merge pull request #501 from KhronosGroup/mkinsner-allocator-may-fail

Improve default allocator wording

(cherry picked from commit 81e9a2f)

adoc/chapters/programming_interface.adoc

@@ -4119,8 +4119,8 @@ 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, it is guaranteed to return
-non-[code]#nullptr# and new memory positions every call.
+For allocations greater than size zero, when successful it is guaranteed to
+return non-[code]#nullptr# and new memory positions every call.
 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>)#.