Harmonize behavior of insert* methods

[dylans]

We currently have a fair bit of inconsistency and confusion with our insert methods, whether they are the version from Editor or Transforms.

I think we should make sure the following happens:

• All insert methods should optionally accept a location
• If no location is provided and there's no selection within the editor, we should either throw or insert at a consistent location (start or end of the document?)
• Update the documentation