rustdoc summaries include parts of doc examples

[bluss]

A documentation comment like the one cited verbatim below, produces a main page summary that includes the first line of the code in the example. This is very surprising since there's an empty line in between, and the code example inserts a large colored box in the overview page (the parent module).

cc @mitaa since it's probably related to PR #31602

/// A tuple or fixed size array that can be used to index an array.
///
/// ```
/// use ndarray::arr2;
///
/// let mut a = arr2(&[[0, 1], [0, 0]]);
/// a[[1, 1]] = 1;
/// assert_eq!(a[[0, 1]], 1);
/// assert_eq!(a[[1, 1]], 1);
/// ```
///
/// **Note** the blanket implementation that's not visible in rustdoc:
/// `impl<D> NdIndex for D where D: Dimension { ... }`