More docs for Timer.cancel()
More docs for Timer.cancel()

Notably, it's worth documenting that calling `cancel()` twice is safe.

Change-Id: I35fdf3f8a810f78f296fc96750ec9ad4d81c6fc7
Reviewed-on: https://dart-review.googlesource.com/c/83744
Commit-Queue: Jonas Jensen <c9327fa216ae2ef84df7c55b809d4ba270141b48@google.com>
Auto-Submit: Jonas Jensen <c9327fa216ae2ef84df7c55b809d4ba270141b48@google.com>
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>

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