Use a single description of our "ISO8601"-ish time formats

See @wesleyac's notes in #274 (closed):

Also, we probably shouldn't describe this as "iso time with no space", since it's not. ISO 8601 supports having the timezone encoded in the format or not, dropping and number of fields as long as the more significant fields remain, decimal fractional seconds, etc. I presume we do not support those things.

My recommendation there was:

we should make sure to update the other places in dir-spec and rend-spec where we use this format (or the version that does have a space) so that they all have a single description. Ideally it would be in one place that we would link to.