Bulk feature request: support timedeltas, milliseconds and below

I would like to use `arrow`'s `humanize()` for the bold part of this:

> Task has completed in **10 seconds**. With 1000 subtasks, that is **10 milliseconds** per subtask.

Currently, this seems hard for two reasons. First, from the start time that I stored, I need to compute the `delta` to `now` myself, divide the `delta` by 1000, subtract it from `now`, and then output that. That seems unncessarily complicated.

Second, everything below one second is "a moment" or something like that.

Or have I missed anything?

Start by reading the existing humanize() entry point and the timedelta handling it exposes. Trace how durations are converted into human-readable units, then define the expected behavior for milliseconds and sub-second values; done means the requested examples work without callers computing their own delta.

Written by the indexing model from the issue text.