org-agenda,org.texi: Fix doc about category filter

* lisp/org-agenda.el (org-agenda-filter-by-category): Documentation of
  the effect of the prefix argument.

* doc/org.texi (Filtering/limiting agenda items): Documentation about
  the effect of the prefix argument when filtering by category.

doc/org.texi

@@ -8616,9 +8616,10 @@ selected.
 @vindex org-agenda-category-filter-preset
 
 Filter the current agenda view with respect to the category of the item at
-point.  Pressing @code{<} another time will remove this filter.  You can add
-a filter preset through the option @code{org-agenda-category-filter-preset}
-(see below.)
+point.  Pressing @code{<} another time will remove this filter.  When called
+with a prefix argument exclude the category of the item at point from the
+agenda.  You can add a filter preset through the option
+@code{org-agenda-category-filter-preset} (see below.)
 
 @orgcmd{^,org-agenda-filter-by-top-headline}
 Filter the current agenda view and only display the siblings and the parent

lisp/org-agenda.el

@@ -7352,8 +7352,11 @@ in the agenda."
 (defvar org-agenda-filtered-by-category nil)
 
 (defun org-agenda-filter-by-category (strip)
-  "Keep only those lines in the agenda buffer that have a specific category.
-The category is that of the current line."
+  "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.
+"
   (interactive "P")
   (if (and org-agenda-filtered-by-category
 	   org-agenda-category-filter)