Adding examples to docs of std::time module

[marcospb19]

And adding missing link to Duration from Instant.

[rust-highfive]

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[marcospb19]

@joshtriplett I found the time module pretty empty, so I tried adding some examples to it, but I'm not sure if these examples should not be inside of Duration and Instant.

[marcospb19]

I also thought about changing some in Duration (here's the current snippet):

use std::time::Duration;

let five_seconds = Duration::new(5, 0);
let five_seconds_and_five_nanos = five_seconds + Duration::new(0, 5);

assert_eq!(five_seconds_and_five_nanos.as_secs(), 5);
assert_eq!(five_seconds_and_five_nanos.subsec_nanos(), 5);

let ten_millis = Duration::from_millis(10);

The reason: I don't think new is as good as from_* to create a Duration.

I would prefer

let five_seconds = Duration::from_secs(5);

Instead of

let five_seconds = Duration::new(5, 0);

[joshtriplett]

@bors r+ rollup

[bors]

📌 Commit faf5985 has been approved by joshtriplett