[PATCH]Add tab completion for foreigh table

Hi

Attached a patch to improve the tab completion for foreigh table.

Also modified some DOC description of ALTER TABLE at [1] in according with CREATE INDEX at [2].

In [1], we use "ALTER INDEX ATTACH PARTITION"
In [2], we use "ALTER INDEX ... ATTACH PARTITION"

I think the format in [2] is better.

[1] https://www.postgresql.org/docs/devel/sql-altertable.html
[2] https://www.postgresql.org/docs/devel/sql-createindex.html

Regards,
Tang