Fix mapcat docstring

Author: sogaiu

src/boot/boot.janet

@@ -1103,8 +1103,9 @@
   and use `array/concat` to concatenate the results, but only if
   no `inds` are provided. Multiple data structures can be handled
   if each `inds` is a data structure and `f` is a function of
-  arity one more than the number of `inds`. The resulting array
-  has a length that is the shortest of `ind` and each of `inds`.
+  arity one more than the number of `inds`. Note that `f` is only
+  applied to values at indeces up to the largest index of the
+  shortest of `ind` and each of `inds`.
   ```
   [f ind & inds]
   (def res @[])