Bug 1248843 - Make it clearer that DebugOnly uses up space even in op…

…timized, non-DEBUG builds. r=Waldo

--HG--
extra : rebase_source : 6de3e55b8a7c4272bec3b5b8b8eb5da7ad78e9cf

mfbt/DebugOnly.h

@@ -30,8 +30,9 @@ namespace mozilla {
  * DebugOnly instances can only be coerced to T in debug builds.  In release
  * builds they don't have a value, so type coercion is not well defined.
  *
- * Note that DebugOnly instances still take up one byte of space, plus padding,
- * when used as members of structs.
+ * NOTE! DebugOnly instances still take up one byte of space, plus padding, even
+ * in optimized, non-DEBUG builds.  Don't use DebugOnly for struct/class members
+ * unless that really doesn't matter to you.
  */
 template<typename T>
 class DebugOnly