Document the usage of `multi`.

[suzdalnitski]

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!

[yordis]

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()

[yordis]

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