[FLINK-12305][docs][table] Improve Table API/SQL time attribute documentation.

[fhueske]

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What is the purpose of the change

• Improve introduction of processing time and event time support of Table API / SQL.

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[hequn8128]

@fhueske Thanks a lot for rephrasing the document. The updates look good to me. I think it is easier to be understood now.

One little suggestion: how about adding a detailed description of how to handle late arriving data, as uses would still interested in it. The description would look like: Proper support for late arriving data has not been added to the Table API / SQL yet. Elements that arrive behind the watermark will be dropped.

Best, Hequn