Model assertion documentation about series needs to be updated

[codersquid]

https://docs.ubuntu.com/core/en/reference/assertions/model

From what I understand, series is meaningless. The verbiage needs to change in the paragraph below the list of fields. If the field is required, there needs to be a comment about it being a meningless magic string so as not to confuse any developer who is trying to build a custom image.