Make admin link more descriptive

In the admin section, it is not immediate clear what the admin link does ("Menu tree" link under "Site configuration".)

This patch:

1. adds a description (essential, IMO),
2. changes the link from "Menu tree" to "Menu trees"; "Menu tree pages" might work too (optional),
3. and moves the under "Site building" which makes it appear just below the "Menus" item (optional).

Take what you like, ignore the rest.