The main description should include:
"You can choose a set of templates saved in your sandbox space by using an appropriate sandbox prefix. For example, if you want to preview a version of Template:Test that you have saved as User:Foo/sandbox/Template:Test, use ''User:Foo/sandbox'' as the prefix."
That's the "templatesandbox-text" message in TemplateSandbox.i18n.php.
Also, right now, 'templatesandbox-page-or-revid' => 'Either page or revid is required.' I'd like to make this more readable by spelling out "page title" and "revision id".