Clarify no-ordering expectation for qlog fields

[rmarx]

As noticed by @LPardue here, we don't actually say anywhere that CDDL field ordering does not matter (it's implied by CDDL and JSON themselves, but probably good to reinforce explicitly).

Not the biggest fan of current prose... tried to find examples in other RFCs but they're all over the place :) So let me know if you'd like to see it worded differently.