Suggestion: Change the output of the possible_values help

[phrohdoh]

My suggestion is to change [values: ...] to [possible values: ...] to match the builder API.

extern crate clap;
use clap::{App, Arg};

fn main() {
    let matches = App::new("clap_issue_1160")
	.arg(Arg::with_name("myarg").possible_values(&["foo", "bar"]))
	.get_matches();
}

$ cargo run -- qux
error: 'qux' isn't a valid value for '<myarg>'
	[values: bar, foo]


USAGE:
    clap_issue_1160 [myarg]

For more information try --help

$ cargo run -- --help
clap_issue_1160

USAGE:
    clap_issue_1160 [myarg]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

ARGS:
    <myarg>     [values: foo, bar]

[kbknapp]

I'm not against this, my only thinking is it makes the lines longer and I try to stick to no wrapping by default if possible on 80-100 width terminals. Having said that, if someone is using several values already, they're probably already wrapping.

I can mark this as mentored and a good first issue if someone wants to take a swing at it.

[kbknapp]

This should be implemented, thanks to @etopiei 👍

[phrohdoh]

Woohoo thanks @etopiei! :D