Documented default values for collapsible are incorrect

We need to clarify the default values for the collapsible widget and the process through which these get changed. 
This arose from a discussion with @arschmitz and @gabrielschulhof and relates to https://github.com/jquery/api.jquerymobile.com/pull/293 and https://github.com/jquery/api.jquerymobile.com/issues/209
