[2906] some more comments about ZoneSpec::index.

src/lib/datasrc/zone_table_accessor_cache.cc

@@ -37,7 +37,7 @@ public:
     }
 
     virtual ZoneSpec getCurrentImpl() const {
-        return (ZoneSpec(0, it_->first));
+        return (ZoneSpec(0, it_->first)); // index is always 0 in this version.
     }
 
     virtual bool isLast() const {

src/lib/datasrc/zone_table_accessor_cache.h

@@ -54,6 +54,9 @@ public:
 
     /// \brief in-memmory cache version of getIterator().
     ///
+    /// From this version of iterator, `ZoneSpec::index` will always be set
+    /// to 0 at the moment.
+    ///
     /// \throw None except std::bad_alloc in case of memory allocation failure
     virtual IteratorPtr getIterator() const;