Update refget naming to refget sequence more thoroughly in the specification

[tcezard]

Prompted by a comment from @jmarshall looked back at the refget sequence specification and realise that there a number of places where we're still using refget in place of refget.sequence
I see a few options:

• Leave the spec as it is and add a Note at the top that state that refget should be understood as refget.sequence in the document.
• Change the specification an replace all mentions of refget to refget.sequence except in the place that would require changes in implementation like default encoding and clarify that both refget and refget.sequence could be used in that case
• Change the specification an replace all mentions of refget to refget.sequence everywhere which will probably require a new version of the specification.

[nsheff]

I made an attempt to do something between 1 and 2 -- I added a note, and also changed the reference in places where it doesn't change the spec, but just clarifies the text.