[SQL] minor - internal doc improvement for InsertIntoTable.

## What changes were proposed in this pull request?
I was reading this part of the code and was really confused by the "partition" parameter. This patch adds some documentation for it to reduce confusion in the future.

I also looked around other logical plans but most of them are either already documented, or pretty self-evident to people that know Spark SQL.

## How was this patch tested?
N/A - doc change only.

Author: Reynold Xin <rxin@databricks.com>

Closes #15749 from rxin/doc-improvement.

1 files changed, 16 insertions, 0 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
index 7a15c2285d..65ceab2ce2 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
@@ -360,6 +360,22 @@ case class OverwriteOptions(
   }
 }
 
+/**
+ * Insert some data into a table.
+ *
+ * @param table the logical plan representing the table. In the future this should be a
+ *              [[org.apache.spark.sql.catalyst.catalog.CatalogTable]] once we converge Hive tables
+ *              and data source tables.
+ * @param partition a map from the partition key to the partition value (optional). If the partition
+ *                  value is optional, dynamic partition insert will be performed.
+ *                  As an example, `INSERT INTO tbl PARTITION (a=1, b=2) AS ...` would have
+ *                  Map('a' -> Some('1'), 'b' -> Some('2')),
+ *                  and `INSERT INTO tbl PARTITION (a=1, b) AS ...`
+ *                  would have Map('a' -> Some('1'), 'b' -> None).
+ * @param child the logical plan representing data to write to.
+ * @param overwrite overwrite existing table or partitions.
+ * @param ifNotExists If true, only write if the table or partition does not exist.
+ */
 case class InsertIntoTable(
     table: LogicalPlan,
     partition: Map[String, Option[String]],