nit: comments to explain why mutator_allocator_remaining_bytes needs to be added back when reset bytes_allocated_since_gc_start

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp

@@ -694,6 +694,7 @@ public:
   // Return an approximation of the bytes allocated since GC start.  The value returned is monotonically non-decreasing
   // in time within each GC cycle.  For certain GC cycles, the value returned may include some bytes allocated before
   // the start of the current GC cycle.
+  // Bytes allocated since gc start includes the total unused bytes in mutator allocator reserved for CAS allocation.
   inline size_t get_bytes_allocated_since_gc_start() const {
     return _mutator_bytes_allocated_since_gc_start;
   }

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -2293,6 +2293,7 @@ void ShenandoahHeap::reset_bytes_allocated_since_gc_start() {
     // Single-gen Shenandoah uses global heuristics.
     unaccounted_bytes = heuristics()->force_alloc_rate_sample(actual_allocated);
   }
+  // Bytes allocated since gc start includes the total unused bytes in mutator allocator reserved for CAS allocation.
   _free_set->reset_bytes_allocated_since_gc_start(unaccounted_bytes + mutator_allocator_remaining_bytes);
 }