Rearrange CLUSTER rules in gram.y.

Rearrange CLUSTER rules in gram.y.

This change moves the unparenthesized syntax for CLUSTER to the end
of the ClusterStmt rules in preparation for a follow-up commit that
will move this syntax to the "Compatibility" section of the CLUSTER
documentation.  The documentation for the CLUSTER syntaxes has also
been consolidated.

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