Add explanatory message for `[#must_use]` on `std::ops::Add` et al

[jruderman]

Given the following code (playground):

use std::ops::Add;

fn main() {
    let mut x = 2_u32;
    x += 3;
    x.add(4);
    println!("{}", x);
}

The current output is:

warning: unused return value of `add` that must be used
 --> src/main.rs:6:5
  |
6 |     x.add(4);
  |     ^^^^^^^^^
  |
  = note: `#[warn(unused_must_use)]` on by default

Ideally the output should include:

  = note: this returns the result of the operation, without modifying the original

Precedent

Many other std methods have this exact must_use message, e.g. (playground):

use std::path::Path;

fn f(p: &Path) { 
    p.to_str();    
}

which gives the warning:

warning: unused return value of `Path::to_str` that must be used
 --> src/lib.rs:4:5
  |
4 |     p.to_str();    
  |     ^^^^^^^^^^^
  |
  = note: this returns the result of the operation, without modifying the original
  = note: `#[warn(unused_must_use)]` on by default

[chenyukang]

@rustbot claim

[chenyukang]

Change this line:

rust/library/core/src/ops/arith.rs

Line 89 in 5c8bff7

#[must_use]

to

 #[must_use = "this returns the result of the operation, \
                  without modifying the original"]

will fix it, we may add attribute value to other trait such as Sub, Mul.
I'm not sure why whether it's necessary to keep it.
@m-ou-se How do you think about it?