Add contextMenuPlugin.submenuTitle to docs and test.

I couldn't actually get the subtitles to appear (even for built-in) submenus, but I figured I'd add this to the docs anyways.

Author: dhughes-xumak

docs/api.md

@@ -221,6 +221,10 @@ Constructs a new JSONEditor.
 
      The callback function when the custom action is clicked. Called with the selected node.
 
+  - `{string} submenuTitle`
+
+     The submenu expander's title (tooltip) text, for example `A longer description of the submenu`.
+
   - `{Object[]} submenu`
 
      Submenu items of the same type as `contextMenuPlugins`.
@@ -249,6 +253,10 @@ Constructs a new JSONEditor.
 
      The callback function when the custom action is clicked. Called with the selected nodes.
 
+  - `{string} submenuTitle`
+
+     The submenu expander's title (tooltip) text, for example `A longer description of the submenu`.
+
   - `{Object[]} submenu`
 
      Submenu items of the same type as `multiContextMenuPlugins`.

test/test_plugins.html

@@ -85,6 +85,7 @@
         text: 'Submenu',
         title: 'Action with submenu',
         className: 'jsoneditor-action2',
+        submenuTitle: 'A title for the submenu expand button',
         submenu: [
           {
             text: 'Submenu 1',