Document that 'schema: :latest' is added to background migration specs

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/background_migrations.md

@@ -301,12 +301,13 @@ It is required to write tests for:
 - The background migration itself.
 - A cleanup migration.
 
-You can use the `:migration` RSpec tag when testing the migrations.
+The `:migration` and `schema: :latest` RSpec tags are automatically set for
+background migration specs.
 See the
 [Testing Rails migrations](testing_guide/testing_migrations_guide.md#testing-a-non-activerecordmigration-class)
 style guide.
 
-When you do that, keep in mind that `before` and `after` RSpec hooks are going
+Keep in mind that `before` and `after` RSpec hooks are going
 to migrate you database down and up, which can result in other background
 migrations being called. That means that using `spy` test doubles with
 `have_received` is encouraged, instead of using regular test doubles, because

doc/development/testing_guide/testing_migrations_guide.md

@@ -158,7 +158,9 @@ end
 
 To test a non-`ActiveRecord::Migration` test (a background migration),
 you will need to manually provide a required schema version. Please add a
-schema tag to a context that you want to switch the database schema within.
+`schema` tag to a context that you want to switch the database schema within.
+
+If not set, `schema` defaults to `:latest`.
 
 Example: