Add :wrap_in_transaction option to explain

[greg-rychlewski]

Closes elixir-ecto/ecto#4641

There were two options discussed:

1. Add this option
2. Don't rollback when analyze: false

I went for the first option for two reasons:

• The transaction code is shared by any non-standard adapter so trying to make less assumptions about what the correct behaviour should be
• It's more flexible allowing the user to not roll back if they want to use analyze: true

[josevalim]

Code wise looks good to me. I wonder if we should call it :transaction or :wrap_in_transaction though, although I am happy to go with what you decide!

[greg-rychlewski]

I like wrap_in_transaction!