libstd: improve os::args() doc

[liigo]

No description provided.

[huonw]

I don't think we have this as a guarantee, unfortunately; see, for example, http://stackoverflow.com/a/2051031/1256624. As an example:

// otherargv.rs

extern crate libc;
use std::ptr;

fn main() {
    let args = std::os::args();
    println!("{}", args);

    match args.len() {
        1 => unsafe {
            // call itself with a custom argv.
            libc::execv("./otherargv\0".as_ptr() as *const i8,
                        ["abc\0".as_ptr() as *const i8,
                         "def\0".as_ptr() as *const i8,
                         ptr::null()].as_mut_ptr());

        },
        2 => unsafe {
            // call itself with an empty argv.
            libc::execv("./otherargv\0".as_ptr() as *const i8,
                        [ptr::null()].as_mut_ptr());
        },
        _ => {}
    }
}

Running ./otherargv gives:

[./otherargv]
[abc, def]
[]

I think this is still a useful addition, but maybe it could be adjusted to something like:

The first element is traditionally the path to the executable, but it can be set to arbitrary text, and it may not even exist, so this property should not be relied upon for security purposes.

[liigo]

@huonw Thank you! I updated and rebased. r?

[liigo]

ping @huonw