Auto login user after password reset via a plugin in Craft3
Those docs were a bit confusing/misleading, so sorry about that. (I don’t even know what it meant by that!)

They’ve been updated for clarity:


  After the user successfully logs in, they will be redirected to the path defined by the setPasswordSuccessPath config setting.


Additionally, the next Craft release (3.1.8) will start automatically logging users in after resetting their password, if the autoLoginAfterAccountActivation config setting is enabled.

So to answer your question, there won’t be any custom plugin/module code needed for this; just update to 3.1.8 when it comes out and enable that config setting.