Add documentation to OffsetDateTime::unix_timestamp about constructing a timestamp-nanosecond pair from an OffsetDateTime

[sorairolake]

That is, the inverse operation of the following example:

time/time/src/offset_date_time.rs

Lines 350 to 362 in 0a50218

	/// If you have a timestamp-nanosecond pair, you can use something along the lines of the
	/// following:
	///
	/// ```rust
	/// # use time::{Duration, OffsetDateTime, ext::NumericalDuration};
	/// let (timestamp, nanos) = (1, 500_000_000);
	/// assert_eq!(
	/// OffsetDateTime::from_unix_timestamp(timestamp)? + Duration::nanoseconds(nanos),
	/// OffsetDateTime::UNIX_EPOCH + 1.5.seconds()
	/// );
	/// # Ok::<_, time::Error>(())
	/// ```
	pub const fn from_unix_timestamp(timestamp: i64) -> Result<Self, error::ComponentRange> {

I think this operation can be written as follows:

use time::{ext::NumericalDuration, OffsetDateTime};

let dt = OffsetDateTime::UNIX_EPOCH + 1.5.seconds();
assert_eq!((dt.unix_timestamp(), dt.nanosecond()), (1, 500_000_000));