[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.

(cherry picked from commit 0ea5d5b2)
Signed-off-by: Reynold Xin <rxin@databricks.com>