[2850] Comment new ZoneTable::destroy() argument

src/lib/datasrc/memory/zone_table.h

@@ -147,8 +147,10 @@ public:
     /// \param ztable A non NULL pointer to a valid \c ZoneTable object
     /// that was originally created by the \c create() method (the behavior
     /// is undefined if this condition isn't met).
+    /// \param unused Ununsed parameter, provided so it's compatible to
+    /// SegmentObjectHolder.
     static void destroy(util::MemorySegment& mem_sgmt, ZoneTable* ztable,
-                        int = 0);
+                        int unused = 0);
 
     /// \brief Return the number of zones contained in the zone table.
     ///