`update` doesn't work with Experimental syntax

`update` does not work with the new Experimental syntax, even though it probably should!

Unfortunately, support for fancier `update`s differs across backends. [MySQL](https://dev.mysql.com/doc/refman/8.0/en/update.html) and [Postgresql](https://www.postgresql.org/docs/9.3/sql-update.html) differ, and [SQLite](https://www.sqlite.org/lang_update.html) appears to mirror Postgres.

As an example, MySQL allows you to specify updates on a bunch of tables in the `FROM` clause, while the other two engines require a single table to be updated (but other tables can be mentioned in the `FROM` for filtering).

The issue names no repository files or tests. Start by comparing Experimental syntax's update support with the linked MySQL, PostgreSQL, and SQLite UPDATE documentation; define backend-specific behavior and tests before considering it done.

Written by the indexing model from the issue text.