[SPARK-19016][SQL][DOC] Document scalable partition handling

## What changes were proposed in this pull request?

This PR documents the scalable partition handling feature in the body of the programming guide.

Before this PR, we only mention it in the migration guide. It's not super clear that external datasource tables require an extra `MSCK REPAIR TABLE` command is to have per-partition information persisted since 2.1.

## How was this patch tested?

N/A.

Author: Cheng Lian <lian@databricks.com>

Closes #16424 from liancheng/scalable-partition-handling-doc.