update doc RE menu

doc/NERD_tree.txt

@@ -575,8 +575,10 @@ The NERD tree has a menu that can be programmed via the an API (see
 |NERDTreeMenuAPI|). The idea is to simulate the "right click" menus that most
 file explorers have.
 
-The script comes with one default menu plugin that adds some basic filesystem
-operations to the menu for creating/deleting/moving/copying files and dirs.
+The script comes with two default menu plugins: exec_menuitem.vim and
+fs_menu.vim. fs_menu.vim adds some basic filesystem operations to the menu for
+creating/deleting/moving/copying files and dirs. exec_menuitem.vim provides a
+menu item to execute executable files.
 
 ==============================================================================
 3. Customisation                                             *NERDTreeOptions*
@@ -928,6 +930,22 @@ The NERD tree script allows you to add custom key mappings and menu items via a
 set of API calls. Any such scripts should be placed in ~/.vim/nerdtree_plugin/
 (*nix) or ~/vimfiles/nerdtree_plugin (windows).
 
+The script exposes some prototype objects that can be used to manipulate the
+tree and/or get information from it: >
+    g:NERDTreePath
+    g:NERDTreeDirNode
+    g:NERDTreeFileNode
+    g:NERDTreeBookmark
+<
+See the code/comments in NERD_tree.vim to find how to use these objects. The
+following code conventions are used:
+    * class members start with a capital letter
+    * instance members start with a lower case letter
+    * private members start with an underscore
+
+See this blog post for more details:
+ http://got-ravings.blogspot.com/2008/09/vim-pr0n-prototype-based-objects.html
+
 ------------------------------------------------------------------------------
 4.1. Key map API                                           *NERDTreeKeymapAPI*