Document how to make a textbox mandatory and add a placeholder (#415)

Author: mawinter69

src/main/resources/io/jenkins/plugins/designlibrary/Inputs/index.jelly

@@ -28,11 +28,12 @@
       <s:group>
         <s:preview>
           <f:entry title="Example" field="example">
-            <f:textbox />
+            <f:textbox placeholder="Enter an example"/>
           </f:entry>
         </s:preview>
         <s:code file="example.jelly" />
       </s:group>
+      <p class="jdl-paragraph">${%textbox.usage}</p>
     </s:section>
 
     <s:section title="${%Search}">

src/main/resources/io/jenkins/plugins/designlibrary/Inputs/index.properties

@@ -12,4 +12,4 @@ syntaxHighlight.description.2=There is support for languages like <code>groovy</
   format (except the start and end bracket) to be passed as CodeMirror option object.
 html.description=To edit html and have a preview you need the <a href="https://plugins.jenkins.io/antisamy-markup-formatter/" target="_blank">OWASP Markup Formatter</a> \
   plugin. Below snippet ensures that when the plugin is not installed that the preview will just print plain text.
-
+textbox.usage=By adding <code>clazz="required"</code> you can mark a textbox as being mandatory.

src/main/webapp/Inputs/example.jelly

@@ -1,3 +1,3 @@
 <f:entry title="Example" field="example">
-  <f:textbox />
+  <f:textbox placeholder="Enter an example"/>
 </f:entry>