'timestep_sampling=flux_shift' can offer quality improvements, but barely mentioned in docs

So I just discovered this Flux (SD3 branch) parameter:
```
--timestep_sampling flux_shift
```
Previously I'd been using
```
--timestep_sampling shift
```
due to:
1) The README.md having `--timestep_sampling shift` in the example training command
2) The extensive diagrams showing the effect of changing the `discrete_flow_shift` value and the effect of different `sigmoid` values - none of which apply to the `--timestep_sampling flux_shift` mode. The space given to these modes in the README.md kind of implies (in my opinion) that these are the modes to use.
3) Almost zero casual references to `flux_shift` on this sd-scripts repository. Searching for 'flux_shift' in the GitHub search bar returns very few results.

But now I've stumbled across this mode and tried it, my sample images are immediately more realistic.

I don't think enough people are aware of this mode and how good it can be, so I'm mostly filing this issue to raise its profile. @kohya-ss , should the README.md be updated to make `flux_shift` be the default in the example training command for Flux?

@recris, we've been talking about how to avoid the vertical lines artifact, and I came across this thread where someone has a similar-looking horizontal line artifact, and claims he 'fixed' it by using `flux_shift`:

https://github.com/kohya-ss/sd-scripts/issues/1948

Worth a try if you have some line artifacts and you currently aren't using this timestep sampling mode?

Start with README.md and search for timestep_sampling, flux_shift, and the Flux training example. Compare the documented modes and diagrams, then update the relevant explanation and example so flux_shift is accurately described; done means readers can understand when it applies and how to enable it.

Written by the indexing model from the issue text.