[SYCL][Fusion][NFC] Comment fusion abortion on inter-fusion dependency (

intel#12678)

Add comment stating event dependencies on any commands part of a
different active fusion leads to the abortion of that fusion.

---------

Signed-off-by: Victor Perez <victor.perez@codeplay.com>

sycl/source/detail/scheduler/graph_builder.cpp

@@ -966,8 +966,8 @@ Scheduler::GraphBuildResult Scheduler::GraphBuilder::addCG(
  ++Ev;
  continue;
  }
- // Handle event dependencies on any commands part of another active
- // fusion.
+ // Event dependencies on commands part of another active fusion are
+ // handled by cancelling fusion in that other queue.
  if (EvDepCmd->getQueue() != Queue && isPartOfActiveFusion(EvDepCmd)) {
  printFusionWarning(
  "Aborting fusion because of event dependency from a "