Update index.md

docs/en/sql-reference/window-functions/index.md

@@ -15,6 +15,7 @@ ClickHouse supports the standard grammar for defining windows and window functio
 | Feature | Support or workaround |
 | --------| ----------|
 | ad hoc window specification (`count(*) over (partition by id order by time desc)`) | yes |
+| expressions involving window functions, e.g. `(count(*) over ()) / 2)` | not supported, wrap in a subquery ([feature request](https://github.com/ClickHouse/ClickHouse/issues/19857)) |
 | `WINDOW` clause (`select ... from table window w as (partiton by id)`) | yes |
 | `ROWS` frame | yes |
 | `RANGE` frame | yes, it is the default |