Add include describing use of rowtime

[bbejeck]

Description

When Flink SQL users have a table without an explicit TS column they can use $rowtime for windowed options as parameter to the DESCRIPTION function.

Staging Docs

• http://kafka-tutorials-staging.s3-website-us-west-2.amazonaws.com/add_statement_about_row_time/create-tumbling-windows/flinksql.html
• http://kafka-tutorials-staging.s3-website-us-west-2.amazonaws.com/add_statement_about_row_time/create-hopping-windows/flinksql.html
• http://kafka-tutorials-staging.s3-website-us-west-2.amazonaws.com/add_statement_about_row_time/create-cumulating-windows/flinksql.html

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