"--attempt-instant-ddl" flag causing some issues with hooks and is incompatible with --execute flag

[YazanNoon]

when I tried to use the --attempt-instant-ddl flag, I faced two issues:

• the on-success hook will not trigger after the migration is done.
• even if I don't use the --execute flag, the migration will still happen anyway.

[hakusaro]

If it's not too much effort @timvaillancourt, I would suggest adding this to the docs around --attempt-instant-ddl, as looking at this, the original PR, and other issues led me to pulling this flag out of our production gh-ost scripts for safety.

I know you're very busy, and this is a low priority task, however.