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

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>