Document the behaviors of Debug v.s. Display for Matrix

[SimonSapin]

Original title: Debug printing of Matrix looks transposed

While looking into #1495 I found that std::fmt::Debug is implemented for Matrix by simply delegating to its storage, which in same cases is eventually [[T; R]; C]. This gives a formatting that looks row-major, even though Matrix is supposed to to be column-major.

As a result:

let matrix = nalgebra::matrix!(
    0, 1, 2;
    10, 11, 12;
    20, 21, 22;
);
println!("{matrix:?}");

Prints:

[[0, 10, 20], [1, 11, 21], [2, 12, 22]]

[PaulXiCao]

I believe this is supposed to show the storage format instead of the actual matrix as a 2d grid which is possible via Display, e.g.

println!("debug:   {matrix:?}");
println!("display: {matrix}");

yields

debug:   [[0, 10, 20], [1, 11, 21], [2, 12, 22]]
display: 
  ┌          ┐
  │  0  1  2 │
  │ 10 11 12 │
  │ 20 21 22 │
  └          ┘

In short, I find the debug output useful and would not know how to make it more obvious that it shows the data layout and not the matrix as row-major vectors.

[SimonSapin]

I didn’t realize that Display did something different. In that case yes, I agree it makes sense to have both. It could use being discussed in docs though