[DOCS] Clarify round mode for format_number & round functions

## What changes were proposed in this pull request?

Updated the description for the `format_number` description to indicate that it uses `HALF_EVEN` rounding. Updated the description for the `round` description to indicate that it uses `HALF_UP` rounding.

## How was this patch tested?

Just changing the two function comments so no testing involved.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Roxanne Moslehi <rmoslehi@palantir.com>
Author: roxannemoslehi <rmoslehi@berkeley.edu>

Closes #17399 from roxannemoslehi/patch-1.