document search disable feature (spin-off of limit on unchecked entries)

doc/man/man5/slapd.conf.5

@@ -430,6 +430,10 @@ If it is set to
 or to the keyword 
 .IR none , 
 no limit is applied (the default).
+If it is set to
+.IR disable ,
+the search is not even performed; this can be used to disallow searches
+for a specific set of users.
 If no flag is set, the value is assigned to the
 .BR soft 
 limit, and the