added and/or clarified various date handler comments

also slightly reformatted a few date handler lines
and fixed one (locale-specific) date handler typo


git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@22671 4ae1f11a-8b86-4847-b8af-ab372f36d1fd