Removed category()/tag() documentation changes

Adding "string" to the possible return types of category()/tag() didn't stop the scrutinizer from raising two new issues, so I've reverted that change. In future tags/categories may be split into separate controllers (it's cleaner that way), so these "issues" may serve as a reminder.

NOTE: This means that the documented return type for rss() is also wrong

code/model/Blog.php

@@ -826,7 +826,7 @@ class Blog_Controller extends Page_Controller
     /**
      * Renders the blog posts for a given tag.
      *
-     * @return null|SS_HTTPResponse|string
+     * @return null|SS_HTTPResponse
      */
     public function tag()
     {
@@ -870,7 +870,7 @@ class Blog_Controller extends Page_Controller
     /**
      * Renders the blog posts for a given category.
      *
-     * @return null|SS_HTTPResponse|string
+     * @return null|SS_HTTPResponse
      */
     public function category()
     {