[Javadoc] Clarify behavior of deprecated CSVFormat#withFirstRecordAsHeader()

[makarandhinge]

Description:
This PR updates the Javadoc of the deprecated method CSVFormat.withFirstRecordAsHeader() to clearly explain its behavior.

Previously, the Javadoc was ambiguous: it suggested that calling this method returns a CSVFormat “equal to this but using the first record as header,” but it did not mention that previously set headers are reset to empty and that skipHeaderRecord is automatically set to true.

This update adds a note to the Javadoc to make the behavior clear for users, without changing any runtime behavior.

No code behavior is changed; this is purely a documentation improvement.

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