Fix mapcat and keep docstrings

[sogaiu]

Sorry, I think I messed up the mapcat and keep docstrings in #1578.

---

For mapcat the docstring said:

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

This is not correct, as shown by this example:

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

This PR suggests instead the text:

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

---

For keep, the docstring said:

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

This is not necessarily correct, as shown by this example:

(keep |(when (pos? (+ ;$&)) $0) [1 2 3] [-1 1 1])
# =>
@[2 3]

This PR suggests instead the text:

The resulting array has a length that is no longer than
the shortest of `ind` and each of `inds`.

---

Hopefully, the above suggestions correct the errors.

(Sorry about the mess up with #1580 as well.)

[bakpakin]

No problem, I appreciate the help with making the docstrings more precise.