runtime(doc): make :h virtcol() more accurate

The current description (especially the "unlimited width" part) is
inaccurate in several ways:
- The size of virtual text can depend on window width. In particular,
  the size of "above" virtual text can be equal to window width.
- A double-width character that doesn't fit adds 1 to the virtual column
  of the following characters.
- The size of 'showbreak' and 'breakindent' is counted.

related: #5713
closes: #18447

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 13e73add6bc366af19f341a5ceb6ed612db92b71..46450685aa4a9f6cdd3f7e5e8b13c13f227d84fe 100644 (file)
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*  For Vim version 9.1.  Last change: 2025 Sep 29
+*builtin.txt*  For Vim version 9.1.  Last change: 2025 Oct 01
 
 
                  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -12346,9 +12346,9 @@ values({dict})                                          *values()*
 
 virtcol({expr} [, {list} [, {winid}]])                 *virtcol()*
                The result is a Number, which is the screen column of the file
-               position given with {expr}.  That is, the last screen position
-               occupied by the character at that position, when the screen
-               would be of unlimited width.  When there is a <Tab> at the
+               position given with {expr}.  That is, the total number of
+               screen cells occupied by the part of the line until the end of
+               the character at that position.  When there is a <Tab> at the
                position, the returned Number will be the column at the end of
                the <Tab>.  For example, for a <Tab> in column 1, with 'ts'
                set to 8, it returns 8. |conceal| is ignored.