document why only "valid domain" format is allowed for "effective domain"

[equalsJeffH]

the rationale is essentially the same as for HSTS, which is documented like so...

4.     HSTS Hosts are identified only via domain names -- explicit IP
       address identification of all forms is excluded.  This is for
       simplification and also is in recognition of various issues with
       using direct IP address identification in concert with PKI-based
       security.

[selfissued]

On the 28-Feb-18, we asked whether #515 closed this. @equalsJeffH to investigate.

[equalsJeffH]

This issue has two parts. PR #515 addressed the first part by making explicit that "IP address identification of all forms is excluded." (thx).

However, the rationale is not stated, e.g.:

This is for simplification and also is in recognition of various issues with using direct IP address identification in concert with PKI-based security.

..which we can simply add to the Notes pr #515 added.