docs, bees.h: stop describing the deleted hash_prefetch thread
Commit "hash: delete the prefetch thread; beesstats becomes a report"
deleted the prefetch thread and turned beesstats.txt into a report, but
three places still describe it as running: the BeesHashTable constructor
and start_threads() doc comments, which promise to start "writeback and
prefetch" threads when start_threads() now starts only the writeback
thread, and docs/wrong.md, which lists hash_prefetch in its example
thread dump and in the table of thread names of note.
An operator reading wrong.md to interpret a THREADS report would look
for a thread that cannot appear.