Fix docstrings of List.append and List.concat

[murmour]

Self-descriptive.

[gasche]

I thought that the complexity annotations where about time, not stack space (but then the one on concat/flatten seems to be stack space), and am unsure whether they should be removed or clarified/fixed.

[murmour]

I thought that the complexity annotations where about time, not stack space

I assumed that "O(...)" in "Tail-recursive O(length of the first argument)." is about space, because it is placed in the same sentence as "Tail-recursive", and because the non-tail-recursive version of append takes exactly as much stack space; but you're right that this also the time complexity! I'll add back the time complexity estimate with a better wording.

As for concat, the complexity comment is a leftover from the OCaml standard library.

[murmour]

I'll add back the time complexity estimate with a better wording.

Done.