Select: clarify the docs for multiple selects

[agcolom]

Add a warning box for the note: Currently jQuery Mobile only supports the multiple attribute on a select with nativeMenu set to false.

And revisit the other mentions of multiple selects

[agcolom]

Actually this is already present in http://api.jquerymobile.com/selectmenu/
"Currently jQuery Mobile only supports the multiple attribute on a select with nativeMenu set to false."

Edit: ok, the note says to add a warning div for the existing note... will do that now.