Update a comment.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41218 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bignum.c

@@ -628,7 +628,9 @@ absint_numwords_generic(size_t numbytes, int nlz_bits_in_msbyte, size_t word_num
  *
  * This function returns ((val_numbits * CHAR_BIT + word_numbits - 1) / word_numbits)
  * where val_numbits is the number of bits of abs(val).
- * If it overflows, (size_t)-1 is returned.
+ *
+ * This function can overflow if _val_ is very big and word_numbits is less than CHAR_BIT.
+ * When overflow occur, (size_t)-1 is returned.
  *
  * If nlz_bits_ret is not NULL and overflow is not occur,
  * (return_value * word_numbits - val_numbits) is stored in *nlz_bits_ret.