There is some documentation on the URLs to which PonyMail responds, though it could be improved.
There does not seem to be any formal documentation of the Permalink formats. In order to ensure that Permalinks really are permanent, once a format is introduced and used in a live site, the format must continue to be supported. So I think it's vital to document the formats. And any changes to the Permalink format need to be supported in a backwards compatible way. i.e. the original Permalink must continue to work even if a new format is introduced.
