include details on where the default marker offset number came from

src/ui/marker.js

@@ -127,6 +127,12 @@ class Marker {
             element.appendChild(svg);
 
             // if no element and no offset option given apply an offset for the default marker
+            // -14 is used as the default marker offset since, the offset relative to center of the marker
+            //
+            // the marker tip is at the center of the shadow ellipse from the default svg
+            // the y value of the center of the shadow ellipse relative to the svg top left is "shadow transform translate-y (29.0) + ellipse cy (5.80029008)"
+            // offset to the svg center "height (41 / 2)" gives (29.0 + 5.80029008) - (41 / 2) and rounded for an integer pixel offset gives 14
+            // negative is used to move the marker up from the center so the tip is at the Marker lngLat
             const defaultMarkerOffset = [0, -14];
             if (!(options && options.offset)) {
                 if (!options) {