Index: share/man/man9/bitset.9
===================================================================
--- share/man/man9/bitset.9
+++ share/man/man9/bitset.9
@@ -325,7 +325,7 @@
 .Fn BIT_FFS_AT
 macro returns the 1-index of the first (lowest) set bit in
 .Fa bitset ,
-which is greater than the given 1-indexed
+which is greater than the given 0-indexed
 .Fa start ,
 or zero if no bits in
 .Fa bitset
Index: sys/sys/bitset.h
===================================================================
--- sys/sys/bitset.h
+++ sys/sys/bitset.h
@@ -222,8 +222,8 @@
 } while (0)
 
 /*
- * Note that `start` and the returned value from __BIT_FFS_AT are
- * 1-based bit indices.
+ * Note that the returned value from __BIT_FFS_AT is a
+ * 1-based bit index, while the `start` argument is 0-based.
  */
 #define	__BIT_FFS_AT(_s, p, start) __extension__ ({			\
 	__size_t __i;							\