Add comment for relinkPrivateKeys + transforms

Author: rreusser

src/plots/plots.js

@@ -811,6 +811,8 @@ plots.supplyDataDefaults = function(dataIn, dataOut, layout, fullLayout) {
                 var expandedTrace = expandedTraces[j];
                 var fullExpandedTrace = plots.supplyTraceDefaults(expandedTrace, cnt, fullLayout, i);
 
+                // relink private (i.e. underscore) keys expanded trace to full expanded trace so
+                // that transform supply-default methods can set _ keys for future use.
                 relinkPrivateKeys(fullExpandedTrace, expandedTrace);
 
                 // mutate uid here using parent uid and expanded index