Improve docstring of `org-agenda-filter-by-category'

lisp/org-agenda.el

@@ -7468,8 +7468,8 @@ With a prefix argument, do so in all agenda buffers."
 (defun org-agenda-filter-by-category (strip)
   "Filter lines in the agenda buffer that have a specific category.
 The category is that of the current line.
-Without prefix argument, keep only the lines of that category.
-With a prefix argument, exclude the lines of that category."
+With a `\\[universal-argument]' prefix argument, exclude the lines of that category.
+When there is already a category filter in place, this command removes the filter."
   (interactive "P")
   (if (and org-agenda-filtered-by-category
 	   org-agenda-category-filter)