"ALTER TEXT SEARCH DICTIONARY foobar" can be followed by an open parenthesis, but that is not offered in tab completion. That is useful, because otherwise I have to look up the docs to see if I need a SET or OPTION(S) or WITH or something before it, just to discover I don't.
The attached one-line patch adds "(".
We can't go beyond that, as available options for each dictionary are not known in advance.
Cheers,
Jeff