IANA instructions cleanup

[bemasc]

It turns out that our IANA instructions contain a few deficiencies:

1. The column named "Format Reference" cannot meaningfully be populated for keys that are reserved (and thus have no format), but reservations can be made by documents, and the registry needs to indicate which document requested the reservation.
2. Adding a "Reference" column in addition to the "Format Reference" is extremely duplicative, as they are the same document in almost all cases.
3. There is no indicated Change Controller, as recommended in RFC 8216.