Point out ambiguous principle.

It's clear when it comes to application state (like user's settings, etc), but not clear when the document content affects the application (i.e. if the user puts the cursor on line 10 of bin X, and opens bin Y, should the cursor be on line 10?).

I think we have to ringmark this ambiguous area and accept (for now) we're not sure, and we always do our best to apply this principle where possible.

Closes #13.