doc/api: Clarify group storage moves API use case

The group storage moves API is used to migrate group wiki repositories
from one storage to another. Projects contained within a group are not
migrated as part of this operation. In the documentation, this is not
immediately clear to the reader and it would be reasonable to assume
that moving a group to a new storage would also move all associated
project repositories.

Clarify the group storage moves API documentation introduction to
explicitly explain that a group's project repositories are not moved
through the use of this API.

doc/api/group_repository_storage_moves.md

@@ -9,9 +9,11 @@ type: reference
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53016) in GitLab 13.9.
 
-Group repositories can be moved between storages. This API can help you when
-[migrating to Gitaly Cluster](../administration/gitaly/index.md#migrate-to-gitaly-cluster), for
-example, or to migrate a [group wiki](../user/project/wiki/group.md).
+Group wiki repositories can be moved between storages. This API can help you, for example,
+[migrate to Gitaly Cluster](../administration/gitaly/index.md#migrate-to-gitaly-cluster)
+or migrate a [group wiki](../user/project/wiki/group.md). This API does not manage
+project repositories in a group. To schedule project moves, use the
+[project repository storage moves API](project_repository_storage_moves.md).
 
 As group repository storage moves are processed, they transition through different states. Values
 of `state` are: