doc: add LITERAL tag to RETURNING - Mailing list pgsql-hackers

Hi,

RETURNING is usually tagged with appropriate tags, such as <LITERAL>, 
but not in the 'query' section of COPY.

https://www.postgresql.org/docs/devel/sql-copy.html

Would it be better to put <LITERAL> here as well?

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Regards,

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Atsushi Torikoshi
NTT DATA Group Corporation