don't use document date from Sink API as creation date but as "date" without precision on created or last modified

as seen in DOXIA-541, the meaning of the date is not defined in Doxia: then it seems it's up to the content writer to choose...
and it was a wrong choice to name the content of date field of the document as dateCreation variable in DOXIA-315 / DOXIASITETOOLS-20 r771801 in 2009

let's start by showing the weak sematic before choosing what to do: creation date, last modification date, or even add some more features like multiple dates (for example "created : last modified")?