Added note to CHANGES.txt about Arena::SpaceAllocated().

CHANGES.txt

@@ -8,6 +8,9 @@ Unreleased Changes (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
   * Fixed -Wtautological-constant-out-of-range-compare warning.
   * DynamicCastToGenerated works for nullptr input for even if RTTI is disabled
   * Arena is refactored and optimized.
+  * Clarified/specified that the exact value of Arena::SpaceAllocated() is an
+    implementation detail users must not rely on. It should not be used in
+    unit tests.
   * Change the signature of Any::PackFrom() to return false on error.
 
   Java