Sync gc_impl.h

Author: peterzhu2118

gc/gc_impl.h

@@ -162,6 +162,8 @@ GC_IMPL_FN bool rb_gc_impl_get_measure_total_time(void *objspace_ptr);
 GC_IMPL_FN unsigned long long rb_gc_impl_get_total_time(void *objspace_ptr);
 GC_IMPL_FN size_t rb_gc_impl_gc_count(void *objspace_ptr);
 GC_IMPL_FN VALUE rb_gc_impl_latest_gc_info(void *objspace_ptr, VALUE key);
+/* A NULL objspace requests cumulative process-wide statistics. Collectors that
+ * do not support this scope must raise NotImplementedError. */
 GC_IMPL_FN VALUE rb_gc_impl_stat(void *objspace_ptr, VALUE hash_or_sym);
 GC_IMPL_FN VALUE rb_gc_impl_stat_heap(void *objspace_ptr, VALUE heap_name, VALUE hash_or_sym);
 GC_IMPL_FN const char *rb_gc_impl_active_gc_name(void);