Clarify README on disable_ddl_transaction!

[jareks-ig]

Hello and first thanks for your great project!

Could you please expand on information in README: If you use disable_ddl_transaction!, no lock timeout will occur?
I don't understand why using disable_ddl_transaction would cancel need for lock timeouts. As I understand regardless if it is used or not, ACCESS EXCLUSIVE lock needs to be obtained on table?

Is this because this gem is designed to work together with PGBouncer in transaction mode?

[sdhull]

We ran into this in production, though I'm not sure if it really had any negative effects in prod as the table in question doesn't have a lot of activity, so I'm still not sure why this particular migration (adding an index concurrently) waited for so long. DB monitoring tool showed the wait event as virtualxid but I'm not sure what that means.