doc: expand comment about placeholder calc point

src/plots/plots.js

@@ -1670,9 +1670,12 @@ plots.doCalcdata = function(gd, traces) {
         }
 
         // Make sure there is a first point.
+        //
         // This ensures there is a calcdata item for every trace,
         // even if cartesian logic doesn't handle it (for things like legends).
-        // Tag this artificial calc point with 'placeholder: true'
+        //
+        // Tag this artificial calc point with 'placeholder: true',
+        // to make it easier to skip over them in during the plot and hover step.
         if(!Array.isArray(cd) || !cd[0]) {
             cd = [{x: false, y: false, placeholder: true}];
         }

src/traces/scattergeo/hover.js

@@ -46,6 +46,9 @@ module.exports = function hoverPoints(pointData) {
             dy = Math.abs(yPx - pos[1]),
             rad = Math.max(3, d.mrc || 0);
 
+        // N.B. d.mrc is the calculated marker radius
+        // which is only set for trace with 'markers' mode.
+
         return Math.max(Math.sqrt(dx * dx + dy * dy) - rad, 1 - 3 / rad);
     }