Added DOC block with description of how the test works

feature-detects/css/nthchild.js

@@ -18,6 +18,14 @@
   "warnings": "Known false negative in Safari 3.1 and Safari 3.2.2"
 }
 !*/
+/* DOC
+
+## Detects support for the ':nth-child()' css pseudo-selector.
+
+###How it works
+5 `<div>` elements with `1px` width is created. Then every other element has its `width` set to `2px`. A Javascript lopp then tests if the `<div>`'s has the expected width using the modulus operator. 
+
+*/
 define(['Modernizr', 'testStyles'], function( Modernizr, testStyles ) {
   testStyles('#modernizr div {width:1px} #modernizr div:nth-child(2n) {width:2px;}', function( elem ) {
     Modernizr.addTest('nthchild', function () {