Document `org-agenda-auto-exclude-function' can select positively

* lisp/org-agneda.el: (org-agenda-auto-exclude-function):  Update
documentation.

lisp/org-agenda.el

@@ -2033,7 +2033,8 @@ estimate."
 The sole argument to the function, which is called once for each
 possible tag, is a string giving the name of the tag.  The
 function should return either nil if the tag should be included
-as normal, or \"-<TAG>\" to exclude the tag.
+as normal, \"-<TAG>\" to exclude the tag, or \"+<TAG>\" to exclude
+lines not carrying this tag.
 Note that for the purpose of tag filtering, only the lower-case version of
 all tags will be considered, so that this function will only ever see
 the lower-case version of all tags."