pgsql: Improve handling of SET ACCESS METHOD for ALTER MATERIALIZED VIE

Improve handling of SET ACCESS METHOD for ALTER MATERIALIZED VIEW

b048326 has added support for SET ACCESS METHOD in ALTER TABLE, but it
has missed a few things for materialized views:
- No documentation for this clause on the ALTER MATERIALIZED VIEW page.
- psql tab completion missing.
- No regression tests.

This commit closes the gap on all the points listed above.

Author: Yugo Nagata
Discussion: /messages/by-id/20220316133337.5dc9740abfa24c25ec9f67f5@sraoss.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/eb8399cf1f3dd8ad02633e3bb84e2289d2debb44

Modified Files
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doc/src/sgml/ref/alter_materialized_view.sgml | 1 +
src/bin/psql/tab-complete.c | 6 +++++-
src/test/regress/expected/create_am.out | 26 ++++++++++++++++++++++++++
src/test/regress/sql/create_am.sql | 10 ++++++++++
4 files changed, 42 insertions(+), 1 deletion(-)