Document the usage of `multi`.

It's not immediately clear what the multi passed to the projection function actually does:

project (%MyEvent{}, _, fn multi -> ... end)

How does using this multi differ from returning a new Ecto.Multi?

project (%MyEvent{}, _, fn -> Ecto.Multi.new() ... end)

Does the multi passed to the projection function already contain some other operations? Does it group some other operations together with the projection?

Thanks!

Does the multi passed to the projection function already contain some other operations?

Yes

commanded-ecto-projections/lib/projections/ecto.ex

Line 70 in 6ff9245

|> Ecto.Multi.run(:verify_projection_version, fn repo, _changes ->

commanded-ecto-projections/lib/projections/ecto.ex

Line 92 in 6ff9245

|> Ecto.Multi.update(:projection_version, changeset, prefix: prefix)

Does it group some other operations together with the projection?

Yes

commanded-ecto-projections/lib/projections/ecto.ex

Line 70 in 6ff9245

|> Ecto.Multi.run(:verify_projection_version, fn repo, _changes ->

commanded-ecto-projections/lib/projections/ecto.ex

Line 92 in 6ff9245

|> Ecto.Multi.update(:projection_version, changeset, prefix: prefix)

How does using this multi differ from returning a new Ecto.Multi?

No differences. That multi is an Ecto.Multi

commanded-ecto-projections/lib/projections/ecto.ex

Line 69 in 6ff9245

Ecto.Multi.new()

@suzdalnitski do you mind creating a PR tackling your concern?