Fixed RSS feed link for category/tag feeds
Fixed RSS feed link for category/tag feeds

- The RSS feed link was always the blog's root URL, even for category/tag feeds. FIXED
- Changed the return type documented for category() and tag() to include string for the case where an RSS feed is returned (addresses the "Scrutinizer" issues)
silverstripe/silverstripe-blog,silverstripe/silverstripe-blog,kinglozzer/silverstripe-blog,kinglozzer/silverstripe-blog,javabrett/silverstripe-blog,javabrett/silverstripe-blog,javabrett/silverstripe-blog