Document invalid characters for namespace prefixes

[jhiemstrawisc]

We have a fed prefix validation function aptly called validateFederationPrefix in server_utils/origin.go that checks whether a provided federation prefix is allowable. Most of our restrictions come from being cautious around characters that carry special meaning in POSIX-like filesystems, or are otherwise reserved as special chars in URLs.

However, we don't document these restrictions. I think the new "choosing a namespace" page is the best place for the new docs.