Reword Windows makefile comments to explain the "help" logic about USE_MULTI

There is logic to auto-enable USE_MULTI if either USE_ITHREADS or USE_IMP_SYS
(which both require USE_MULTI) are enabled, but this is not mentioned in
the comments. Explain it so that users don't get confused when trying to
disable USE_MULTI.

Reported in [perl #121494].