Auto merge of #12526 - kpreid:patch-2, r=blyxyas

Mention `size_hint()` effect in `flat_map_option` lint documentation.

The previous documentation for `flat_map_option` mentioned only readability benefits, but there is also at least one performance benefit: the `size_hint()` upper bound is preserved, whereas `flat_map().size_hint()` is always `(0, None)`.

Program demonstrating this difference:

```rust
fn main() {
    let evens = |i| if i % 2 == 0 { Some(i) } else { None };

    dbg!(
        [1, 2, 3].iter().flat_map(evens).size_hint(),
        [1, 2, 3].iter().filter_map(evens).size_hint(),
    );
}
```

changelog: [`flat_map_option`]: Mention the benefit to `size_hint()`.

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