please document that (and how) criterion runs GC

I was running a benchmark with `+RTS -Sstderr` (trying to find a good `-A..` value)

I was surprised to see heavy garbage collection going on - since I was expecting that my function does little allocation, and I put large `+RTS -M.. -H..` values.

Then I read #257 (etc.) and deduced that `criterion` runs the collector. I understand the motivation - but it's not quite clear (to me) from the text at https://hackage.haskell.org/package/criterion-1.6.0.0/docs/Criterion-Main.html, where "garbage collection" is only mentioned in connection to "environments" - but my (very simple) benchmark does not use them.