Add xqdoc documentation to modules/app.xql

[joewiz]

As originally reported by @duncdrum at eXist-db/function-documentation#21.

Without xqdoc documentation, the function documentation for the app.xql module as shown at http://exist-db.org/exist/apps/fundocs/view.html?uri=http://exist-db.org/xquery/app&location=/db/apps/public-repo/modules/app.xql&details=true is not particularly useful.