--- a/doc/customizing.txt	Mon May 10 23:11:31 2021 -0400
+++ b/doc/customizing.txt	Tue May 11 11:32:26 2021 +0200
@@ -2985,6 +2985,28 @@
             ``issue.filter(filterspec={"priority": "1",
             'messages.creation' : '.-1w;'}, sort=[('activity', '+')])``
 
+            Note that when searching for Link and Multilink values, the
+            special value '-1' searches for empty Link or Multilink
+            values. For both, Links and Multilinks, multiple values
+            given in a filter call are combined with 'OR' by default.
+            For Multilinks a postfix expression syntax using negative ID
+            numbers (as strings) as operators is supported. Each
+            non-negative number (or '-1') is pushed on an operand stack.
+            A negative number pops the required number of arguments from
+            the stack, applies the operator, and pushes the result. The
+            following operators are supported:
+	    - '-2' stands for 'NOT' and takes one argument
+	    - '-3' stands for 'AND' and takes two arguments
+	    - '-4' stands for 'OR' and takes two arguments
+	    Note that this special handling of ID arguments is applied only
+	    when a negative number smaller than -1 is encountered as an ID
+	    in the filter call. Otherwise the implicit OR default
+            applies.
+	    Examples of using Multilink expressions would be
+	    - '1', '2', '-4', '3', '4', '-4', '-3'
+	      would search for IDs (1 or 2) and (3 or 4)
+	    - '-1' '-2' would search for all non-empty Multilinks
+
 filter_sql  **Only in SQL backends**
 
             Lists the items that match the SQL provided. The SQL is a