Document date() API as *last modified* or *created* date

as discussed on maven-dev mailing list: http://mail-archives.apache.org/mod_mbox/maven-dev/201603.mbox/%3C44172669.gIOaBk5Zlx%40herve-desktop%3E

Doxia Sink API has a date() method, without telling which date it is:

• document created date
• or document last modified date

code in site renderer uses the date as creation date, but people use the field in apt as last modified date

then we should document the field with this question: at Doxia level, it is not interpreted, and perhaps we should not interpret it at Doxia Siterenderer either...