Reference iOS Safari temporal input bug in explanatory Less comment

Refs #11266, #13098, #15415

[skip sauce]
[skip validator]

less/forms.less

@@ -170,7 +170,7 @@ input[type="search"] {
 // In Mobile Safari, setting `display: block` on temporal inputs causes the
 // text within the input to become vertically misaligned. As a workaround, we
 // set a pixel line-height that matches the given height of the input, but only
-// for Safari.
+// for Safari. See https://bugs.webkit.org/show_bug.cgi?id=139848
 
 @media screen and (-webkit-min-device-pixel-ratio: 0) {
   input[type="date"],