[PATCH] explain tup_fetched/returned in monitoring-stats

I'm making some changes to a program that, among other things, reports
tup_fetched/tup_returned as if it were a cache hit rate, analogous to
blks_hit/blks_fetched.

The documentation didn't help me to understand if that was appropriate,
so I looked at the source and asked on IRC. It seems I'm not the first
person to be confused by these descriptions, so here's a tiny patch to
clarify the meaning of fetched and returned.

-- Abhijit