[NNTP] Article number wording

[Forrest J. Cavalier III]

Russ Allbery wrote:

> +   This standard imposes no maximum limit on article numbers other than
> +   the maximum length of an NNTP command.  Implementors are encouraged
> +   to support arbitrarily large article numbers and SHOULD use at least
> +   the largest integer type their software can support.  However, note
> +   that many widely deployed NNTP clients cannot handle article numbers
> +   larger than 2,147,483,647 or 4,294,967,295 and article numbers higher
> +   than that are likely to cause interoperability problems.

Yes, as an implementor, I'd read that as indicating bignums.  It doesn't
give any indication of Steve Walker's implementation being in the wild.

How about....

+   This standard imposes no maximum limit on article numbers other than
+   the maximum length of an NNTP command.  Note that many widely deployed
+   NNTP clients do not handle article numbers larger than 2,147,483,647 or
+   4,294,967,295, and no widely known clients or servers handle article
+   numbers larger than 18,446,744,073,709,551,615.

Check that 2^64 -1.  I don't trust that I or the calculator did that correctly.

I wouldn't mind seeing 2^31, etc expressions instead of the numbers.  But
the format should match other RFCs.