docs(booleanAttrs): improve parameter docs for boolean attributes

src/ng/directive/booleanAttrs.js

@@ -140,7 +140,8 @@
     </doc:example>
  *
  * @element INPUT
- * @param {expression} ngDisabled Angular expression that will be evaluated.
+ * @param {expression} ngDisabled If the {@link guide/expression expression} is truthy, 
+ *     then special attribute "disabled" will be set on the element.
  */
 
 
@@ -170,7 +171,8 @@
     </doc:example>
  *
  * @element INPUT
- * @param {expression} ngChecked Angular expression that will be evaluated.
+ * @param {expression} ngChecked If the {@link guide/expression expression} is truthy, 
+ *     then special attribute "checked" will be set on the element.
  */
 
 
@@ -206,7 +208,8 @@
      </doc:example>
  *
  * @element SELECT
- * @param {expression} ngMultiple Angular expression that will be evaluated.
+ * @param {expression} ngMultiple If the {@link guide/expression expression} is truthy, 
+ *     then special attribute "multiple" will be set on the element.
  */
 
 
@@ -236,7 +239,8 @@
     </doc:example>
  *
  * @element INPUT
- * @param {string} expression Angular expression that will be evaluated.
+ * @param {string} expression If the {@link guide/expression expression} is truthy, 
+ *     then special attribute "readonly" will be set on the element.
  */
 
 
@@ -269,7 +273,8 @@
     </doc:example>
  *
  * @element OPTION
- * @param {string} expression Angular expression that will be evaluated.
+ * @param {string} expression If the {@link guide/expression expression} is truthy, 
+ *     then special attribute "selected" will be set on the element.
  */