…adoc

## What changes were proposed in this pull request?

Use recommended values for row boundaries in Window's scaladoc, i.e. `Window.unboundedPreceding`, `Window.unboundedFollowing`, and `Window.currentRow` (that were introduced in 2.1.0).

## How was this patch tested?

Local build

Author: Jacek Laskowski <jacek@japila.pl>

Closes #17417 from jaceklaskowski/window-expression-scaladoc.