[SPARK-15533][SQL] Deprecate Dataset.explode

## What changes were proposed in this pull request?

This patch deprecates `Dataset.explode` and documents appropriate workarounds to use `flatMap()` or `functions.explode()` instead.

## How was this patch tested?

N/A

Author: Sameer Agarwal <sameer@databricks.com>

Closes #13312 from sameeragarwal/deprecate.