patch 9.0.1717: virtcol2col returns last byte of a multi-byte char

Problem: virtcol2col returns last byte of a multi-byte char
Solution: Make it return the first byte for a multi-byte char

closes: #12786
closes: #12799

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
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.