Add more information to E0599 for small changes that would make the trait bounds be satisfied

[estebank]

Given

use std::io::{BufRead, BufReader, Read, Write};

fn issue_81421<T: Read + Write>(mut stream: T) {
    let initial_message = format!("Hello world");
    let mut buffer: Vec<u8> = Vec::new();
    let bytes_written = stream.write_all(initial_message.as_bytes());
    let flush = stream.flush();

    loop {
        let mut stream_reader = BufReader::new(&stream);
        //~^ ERROR the trait bound `&T: std::io::Read` is not satisfied [E0277]
        //~| HELP consider removing the leading `&`-reference
        //~| HELP consider changing this borrow's mutability
        stream_reader.read_until(b'\n', &mut buffer).expect("Reading into buffer failed");
        //~^ ERROR the method `read_until` exists for struct `BufReader<&T>`,
    }
}

fn main() {}

we currently emit

error[E0599]: the method `read_until` exists for struct `BufReader<&T>`, but its trait bounds were not satisfied
  --> $DIR/suggest-change-mut.rs:16:23
   |
LL |         stream_reader.read_until(b'\n', &mut buffer).expect("Reading into buffer failed");
   |                       ^^^^^^^^^^ method cannot be called on `BufReader<&T>` due to unsatisfied trait bounds
   | 
  ::: $SRC_DIR/std/src/io/buffered/bufreader.rs:LL:COL
   |
LL | pub struct BufReader<R> {
   | ----------------------- doesn't satisfy `BufReader<&T>: BufRead`
   |
   = note: the following trait bounds were not satisfied:
           `&T: std::io::Read`
           which is required by `BufReader<&T>: BufRead`

It would be nice if we mentioned anywhere that BufReader<&mut T>: BufRead is satisfied.

CC #81990

[estebank]

Current output:

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the trait bound `&T: std::io::Read` is not satisfied
  --> src/main.rs:12:48
   |
12 |         let mut stream_reader = BufReader::new(&stream);
   |                                 -------------- ^^^^^^^ the trait `std::io::Read` is not implemented for `&T`
   |                                 |
   |                                 required by a bound introduced by this call
   |
note: required by a bound in `BufReader::<R>::new`
  --> /rustc/32303b219d4dffa447aa606bc11c7a648f44a862/library/std/src/io/buffered/bufreader.rs:72:5
help: consider removing the leading `&`-reference
   |
12 -         let mut stream_reader = BufReader::new(&stream);
12 +         let mut stream_reader = BufReader::new(stream);
   |
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
5  | fn issue_81421<T: Read + Write>(mut stream: T) where &T: std::io::Read {
   |                                                +++++++++++++++++++++++
help: consider changing this borrow's mutability
   |
12 |         let mut stream_reader = BufReader::new(&mut stream);
   |                                                ~~~~

error[[E0599]](https://doc.rust-lang.org/nightly/error_codes/E0599.html): the method `read_until` exists for struct `BufReader<&T>`, but its trait bounds were not satisfied
  --> src/main.rs:16:23
   |
16 |         stream_reader.read_until(b'\n', &mut buffer).expect("Reading into buffer failed");
   |                       ^^^^^^^^^^ method cannot be called on `BufReader<&T>` due to unsatisfied trait bounds
  --> /rustc/32303b219d4dffa447aa606bc11c7a648f44a862/library/std/src/io/buffered/bufreader.rs:50:1
   |
   = note: doesn't satisfy `BufReader<&T>: BufRead`
   |
   = note: the following trait bounds were not satisfied:
           `&T: std::io::Read`
           which is required by `BufReader<&T>: BufRead`