Tie help block to input with aria-describedby

docs/_jade/customizer-variables.jade

@@ -13,11 +13,13 @@ each section in sections
             div.clearfix
           div.col-xs-4
             label(for="input-" + variable.name)= variable.name
+            - var helpId = "help-block-" + variable.name
             input.form-control(
               id="input-" + variable.name
               type="text"
+              aria-describedby=variable.docstring ? helpId : undefined
               value=variable.defaultValue
               data-var=variable.name)
             if variable.docstring
-              p.help-block!= variable.docstring.html
+              p.help-block(id=helpId)!= variable.docstring.html
 // NOTE: DO NOT EDIT THE PRECEDING SECTION DIRECTLY! It is autogenerated via the `build-customizer-html` Grunt task using the customizer-variables.jade template.