runtime(doc): clarify separator cell on status line rows

runtime(doc): clarify separator cell on status line rows

- Expand hl-VertSplit / hl-VertSplitNC in syntax.txt to spell out which
  character (space vs 'fillchars' "vert") and which highlight group
  (StatusLine / StatusLineNC / VertSplit / VertSplitNC) are used at the
  separator cell on each kind of screen row.
- Add cross references from hl-StatusLine and hl-StatusLineNC to
  hl-VertSplit / hl-VertSplitNC.

The behavior itself is unchanged — see v9.2.0349 (c72196529) — but the
asymmetry reported in #20089 surprised users, so this aims to make the
spec discoverable from the highlight group docs.

closes: #20101

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>