Improve ALTER TABLE synopsis
Improve ALTER TABLE synopsis

Add into the ALTER TABLE synopsis the definition of
partition_bound_spec, column_constraint, index_parameters and
exclude_element.

Initial patch by Lætitia Avrot, with further improvements by Amit
Langote and Thomas Munro.

Discussion: https://postgr.es/m/flat/27ec4df3-d1ab-3411-f87f-647f944897e1%40lab.ntt.co.jp

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