define "registrable domain" "registered domain" on publicsuffix.org/list

[equalsJeffH]

the Algorithm section of https://publicsuffix.org/list/ contains this step:
7. The registered or registrable domain is the public suffix plus one additional label.
..but "registered domain" and "registrable domain" are not defined.

Discussion with <team@publicsuffix.org> yielded these definitions:

7. The registered (or registrable) domain is the public suffix plus one additional label, where:

• A registered domain is a domain that is registered, which is a process defined within the domain name system related to IANA functions, ICANN, and domain name registries.
• A registrable domain is a formally valid domain name that can be registered at a registry, i.e., it is not presently registered.

Perhaps the above definitions (or refinements thereof) could be added to https://publicsuffix.org/list/ ? various folks are using the term "registrable domain" in various specs where they likely mean "registered domain" (but the original text of step 7 made it easier to latch onto the term "registrable domain" by default).

Please see also PR #2.

[this issue supersedes publicsuffix/list#236]

[annevk]

See also discussion at publicsuffix/list#27.