Clarified the semantics of the Offset type.

Change-Id: Iccc36d24321ac4d556692ac715c0cc69a2c9e09e
Tested: on Linux.

include/flatbuffers/flatbuffers.h

@@ -158,11 +158,13 @@ typedef std::unique_ptr<uint8_t, std::function<void(uint8_t * /* unused */)>>
 #endif
 
 // Wrapper for uoffset_t to allow safe template specialization.
+// Value is allowed to be 0 to indicate a null object (see e.g. AddOffset).
 template<typename T> struct Offset {
   uoffset_t o;
   Offset() : o(0) {}
   Offset(uoffset_t _o) : o(_o) {}
   Offset<void> Union() const { return Offset<void>(o); }
+  bool IsNull() const { return !o; }
 };
 
 inline void EndianCheck() {
@@ -855,7 +857,7 @@ FLATBUFFERS_FINAL_CLASS
   }
 
   template<typename T> void AddOffset(voffset_t field, Offset<T> off) {
-    if (!off.o) return;  // An offset of 0 means NULL, don't store.
+    if (off.IsNull()) return;  // Don't store.
     AddElement(field, ReferTo(off.o), static_cast<uoffset_t>(0));
   }