docs: expand "Threads and load management" to suggest not running bees so much

docs: expand "Threads and load management" to suggest not running bees so much

One of the more obvious ways to reduce bees load is to simply not run
it all the time.  Explicitly state using maintenance windows as a load
management option.

SIGUSR1 and SIGUSR2 should have been documented somewhere else before now.
Better late than never.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>