Clarify how Rust treats backslashes at end of line in string literals

[tbu-]

Rust differs in that behavior from C: In C, the newline escapes are resolved
before anything else, and in Rust this depends on whether the backslash is
escaped itself.

A difference can be observed in the following two programs:

#include <stdio.h>
int main()
{
    printf("\\
n\n");
    return 0;
}

fn main() {
    println!("\\
n");
}

The first program prints two newlines, the second one prints a backslash, a
newline, the latin character n and a final newline.

[rust-highfive]

r? @steveklabnik

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[steveklabnik]

@bors: r+ rollup

thanks!

[bors]

📌 Commit ce6baa7 has been approved by steveklabnik