8272725: G1: add documentation on needs_remset_update_t vs bool

Reviewed-by: tschatzl, iwalulya

src/hotspot/share/gc/g1/g1HeapRegionAttr.hpp

@@ -39,6 +39,8 @@ struct G1HeapRegionAttr {
 #else
   typedef int8_t region_type_t;
 #endif
+  // _needs_remset_update_t is essentially bool, but we need precise control
+  // on the size, and sizeof(bool) is implementation specific.
   typedef uint8_t needs_remset_update_t;
 
 private: