[2108] Fix doc comment for addFromLoad()

addFromLoad() used to throw before, but now the called code itself
throws.  add() should never return anything except SUCCESS, so if it
returns anything else, we assert.

src/lib/datasrc/memory/memory_client.cc

@@ -529,8 +529,7 @@ public:
     }
 
     /*
-     * Same as above, but it checks the return value and if it already exists,
-     * it throws.
+     * Wrapper around above.
      */
     void addFromLoad(const ConstRRsetPtr& set,
                      const Name& zone_name, ZoneData* zone_data)