patch 9.0.2032: cannot get mouse click pos for tab or virt text

Problem:  Cannot accurately get mouse clicking position when clicking on
          a TAB or with virtual text.
Solution: Add a "coladd" field to getmousepos() result.

closes: #13335

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index f621fc0..a9bba86 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -3928,6 +3928,8 @@
 			wincol		column inside "winid"
 			line		text line inside "winid"
 			column		text column inside "winid"
+			coladd		offset (in screen columns) from the
+					start of the clicked char
 		All numbers are 1-based.
 
 		If not over a window, e.g. when in the command line, then only