diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 96db898..2f6961d 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -10347,6 +10347,9 @@
 		{lnum}, then the byte index of the character at the last
 		virtual column is returned.
 
+		For a multi-byte character, the column number of the first
+		byte in the character is returned.
+
 		The {winid} argument can be the window number or the
 		|window-ID|. If this is zero, then the current window is used.