Fix mapcat docstring

[sogaiu]

Sorry, I think I messed up the mapcat docstring.

The version that was merged recently says:

The resulting array has a length that is the shortest
of `ind` and each of `inds`.

but this doesn't necessarily hold, e.g.

(mapcat |(tuple ;$&) [:a :b] [:x :y :z] [0 1])
# =>
@[:a :x 0 :b :y 1]

I've substituted the following sentence in this PR:

Note that `f` is only applied to values at indeces up
to the largest index of the shortest of `ind` and each
of `inds`.

Hopefully this is more correct (^^;