std: time: Avoid to use "was created" in elapsed() description

[fbq]

".. since this instant was created" is inaccurate and misleading, consider the following case:

	let i1 = Instant::now(); // i1 is created at T1
	let i2 = i1 + Duration::from_nanos(0); // i2 is "created" at T2
	i2.elapsed(); // at T3

Per the current description, elapsed() at T3 should return T3 - T2?

To avoid the inaccuracy, removes the "was created" in the description of {Instant,SystemTime}::elapsed().
And since these types represent times, it's OK to use prepostions with them, e.g. "since this instant".

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[thomcc]

Hm, good point. Thanks.

@bors r+ rollup

[bors]

📌 Commit b54a5fd has been approved by thomcc

It is now in the queue for this repository.