[FLINK-17931] Document fromValues clause in Table API

[dawidwys]

What is the purpose of the change

Add documentation on the fromValues clause.

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• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
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• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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[aljoscha]

Good work, I had some slight comments about wording. I only commented on one part, the changes should of course be applied to all four parts the same way. Then it's good to merge.

[aljoscha]

Suggested change

	<p>You can use a `row(...)` expression to create a composite rows:</p>
	<p>You can use a `row(...)` expression to create composite rows:</p>

[aljoscha]

Suggested change

	|-- f1: VARCHAR(5) NOT NULL // original types CHAR(3) and CHAR(5) are generalized
	// to VARCHAR(5). It uses VARCHAR instead of CHAR so that
	// no padding is applied
	|-- f1: VARCHAR(5) NOT NULL // original types CHAR(3) and CHAR(5) are generalized
	// to VARCHAR(5). VARCHAR is used instead of CHAR so that
	// no padding is applied