Document quirks of deactivate-only mode

[tlater-famedly]

The deactivate-only mode is implemented specifically for use cases where users manually handle user creation/editing, but want to have automated deactivation, to ensure disabled users are always synced correctly.

Due to the limitations of how our "cache" works, this will create a cache incompatible with non-deactivate-only mode, as it will consider all seen users synced, regardless of whether they were only disabled or not.

Long-term we need to fix "caching" (#53), but we should document the current behavior to be clear.

[jannden]

Does this documentation fulfill this?

[tlater-famedly]

Yep, thanks @jannden !