Is your feature request related to a problem? Please describe.
If manage_relationship is called without opts it will do nothing. Ash could warn about this.

Describe the solution you'd like
Add a warning if manage_relationship is a no-op.

Describe alternatives you've considered
Remove the default value of the opts argument; likely not possible due to backwards compat.

Express the feature either with a change to resource syntax, or with a change to the resource interface

Ash.Changeset.manage_relationship(changeset, relationship, value)
# warning: this does nothing. Needs `type: :append` or some other configuration.

Additional context