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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