clarify serve-stale documentation, and add a floor for max-stale-ttl

clarify serve-stale documentation, and add a floor for max-stale-ttl

- added a 1-second floor to max-stale-ttl similar to stale-answer-ttl;
  if set to 0, it will be silently updated to 1.
- fixed the ARM entry on max-stale-ttl, which incorrectly suggested that
  the default was 0 instead of 1 week.
- clarified rndc serve-stale documentation.