Clarify the prefetch multiplier

For years, I thought CELERYD_PREFETCH_MULTIPLIER turned off prefetching, and I had a lot of weird problems.  Today I finally figured out 1 means disabled, 0 means PREFETCH ALL THE THINGS.  This fixes the documentation bug.
Merge pull request #2167 from kevin1024/patch-1

Documentation fix: Clarify the prefetch multiplier