Clear expectations of sync modes

**Describe the bug**
Ssosync unexpectedly creates and deletes users when sync_method is set to _groups_.

**Expected behavior**
Sync_method _groups_ should only create, delete and edit group membership, and not create or delete Identity Center users. At minimum it is not clear in documentation, and assumption could be made that _groups_ method will sync only groups and their membership, and _users_groups_ would create and delete also users.

**Additional context**
In my case I've already configured regular SCIM provisioning from Google Workspace to AWS IAM Identity Center. Having ssosync touching these users feels a bit redundant.