clarify nonce vs validity period

[mcr]

> 12) Line 341: Pardon my ignorance here, but could you elaborate on the
> relationship between nonce and validity period?  Since the term is
> about "nonceless audit voucher", it may be better to state that because
> there is no validity period associated with the voucher, a nonce is not
> require.  It would also be good to spell out what "without a validity
> period" means: does it mean the audit voucher is presumed perpetually
> valid?