Clarify behavior of map/filter

[Stebalien]

Apparently iter::Map and iter::Filter implement DoubleEndedIterator. This could lead to unexpected behavior:

let counter = iter::repeat(()).map({
    let mut ctr = 0;
    move |_| {
        ctr += 1;
        ctr
    }
});

let counter2 = counter.reverse(); // Doesn't reverse anything.

These adapters are only really reversible when passed closure is side-effect free. Unfortunately, we can't simply say that map calls the passed closure on each element yielded because counter.last() != counter.reverse().next().

[steveklabnik]

/cc @rust-lang/libs

[Kimundi]

I don't think we need to change anything here apart from maybe improving the docs by pointing out this pitfall.

The behavior of last(), reverse() etc is well specified, so different behavior if combined with side-effecting closures should be too.