Clarify how INT_N truncates values

We've assumed that INT_1(x), where x is an integer, means x % 256. We should make this explicit.

https://gitweb.torproject.org/torspec.git/tree/proposals/224-rend-spec-ng.txt#n146

https://github.com/eewayhsu/tor-project/commit/443a3bd64096b36bbfe711ca9b43c52ccb5442dc