docs(input): document the md-select-on-focus directive.

Closes #7124

  Closes #7271

src/components/input/input.js

@@ -548,6 +548,39 @@ function placeholderDirective($log) {
   }
 }
 
+/**
+ * @ngdoc directive
+ * @name mdSelectOnFocus
+ * @module material.components.input
+ *
+ * @restrict A
+ *
+ * @description
+ * The `md-select-on-focus` directive allows you to automatically select the element's input text on focus.
+ *
+ * <h3>Notes</h3>
+ * - The use of `md-select-on-focus` is restricted to `<input>` and `<textarea>` elements.
+ *
+ * @usage
+ * <h3>Using with an Input</h3>
+ * <hljs lang="html">
+ *
+ * <md-input-container>
+ *   <label>Auto Select</label>
+ *   <input type="text" md-select-on-focus>
+ * </md-input-container>
+ * </hljs>
+ *
+ * <h3>Using with a Textarea</h3>
+ * <hljs lang="html">
+ *
+ * <md-input-container>
+ *   <label>Auto Select</label>
+ *   <textarea md-select-on-focus>This text will be selected on focus.</textarea>
+ * </md-input-container>
+ *
+ * </hljs>
+ */
 function mdSelectOnFocusDirective() {
 
   return {