Clarify what codes can be shortened in the spec

[drinckes]

(From #273 )

Review specification to clarify that all the following are permitted:

Should shortening of codes with only eight digits be allowed? E.g. 8FVC9G8F+
Should shortening of codes with six digits be allowed? E.g., 8FVC9G00+
Should shortening of codes with two or four digits be allowed? E.g., 8FVC0000+

I think we should also specify that padding after the plus e.g. +00G, is not permitted.