diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 661876b..e9bdad2 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -3551,6 +3551,8 @@
 		cmdline		|cmdline-completion| result
 		compiler	compilers
 		cscope		|:cscope| suboptions
+		custom,{func}	custom completion, defined via {func}
+		customlist,{func} custom completion, defined via {func}
 		diff_buffer     |:diffget| and |:diffput| completion
 		dir		directory names
 		environment	environment variable names