bitset: __BIT_FFS_AT(): Fix herald comment, take 2

Remove the reference to the nonexistent 'end' parameter.  While here,
rephrase a bit.

I did the initial comment fix (commit "bitset: Fix __BIT_FFS_AT()'s
herald comment", f3ab0d8) as part of an experiment introducing
macros to operate on ranges of bits in a bitset and subject to
a predicate (a generalization of some code used in some pending
modifications of the ULE scheduler), which was finally ditched as being
too verbose and impractical to use.  I however then forgot to remove the
reference to 'end'.

No functional change.

Noted by:       emaste
Approved by:    emaste (mentor)
MFC after:      3 days
MFC with:       f3ab0d8
Sponsored by:   The FreeBSD Foundation

sys/sys/bitset.h

@@ -232,9 +232,8 @@
 } while (0)
 
 /*
- * 'start' and 'end' are 0-based bit (runtime) indices. Note that, as for ffs(),
- * the returned index is 1-based, 0 being reserved to indicate that no bits are
- * set.
+ * 'start' is a 0-based bit index.  By contrast, and as for ffs(), the returned
+ * index is 1-based, 0 being reserved to indicate that no bits are set.
  */
 #define	__BIT_FFS_AT(_s, p, start) __extension__ ({			\
 	__size_t __i;							\