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

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.

N/A.

Author: Cheng Lian <e6f14efef9fae0ae8f92d04007faea68035873ce@databricks.com>

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

(cherry picked from commit 871f6114ac0075a1b45eda8701113fa20d647de9)
Signed-off-by: Cheng Lian <e6f14efef9fae0ae8f92d04007faea68035873ce@databricks.com>

big-pegasus/spark,big-pegasus/spark,big-pegasus/spark,big-pegasus/spark,SnappyDataInc/spark,SnappyDataInc/spark,big-pegasus/spark,SnappyDataInc/spark,big-pegasus/spark,SnappyDataInc/spark,big-pegasus/spark,SnappyDataInc/spark,SnappyDataInc/spark,SnappyDataInc/spark