pgsql: Support parenthesized syntax for CLUSTER without a table name.

Support parenthesized syntax for CLUSTER without a table name.

b5913f6120 added a parenthesized syntax for CLUSTER, but it
requires specifying a table name. This is unlike commands such as
VACUUM and ANALYZE, which do not require specifying a table in the
parenthesized syntax. This change resolves this inconsistency.
This is preparatory work for a follow-up commit that will move the
unparenthesized syntax to the "Compatibility" section of the
CLUSTER documentation.

Reviewed-by: Melanie Plageman, Michael Paquier
Discussion: https://postgr.es/m/CAAKRu_bc5uHieG1976kGqJKxyWtyQt9yvktjsVX%2Bi7NOigDjOA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cdaedfc96d1488069518d42479125ceb9a5e67d0

Modified Files
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doc/src/sgml/ref/cluster.sgml | 2 +-
src/backend/parser/gram.y | 12 +++++++++++-
2 files changed, 12 insertions(+), 2 deletions(-)