`Combiner.sigma_clipping`: `low_thresh`/`high_thresh` docstring no longer matches behaviour (`None` means 3σ, negatives are not `abs()`-ed)

The `sigma_clipping` docstring (`ccdproc/combiner.py`, currently lines 443–452) still says:

> low_thresh : positive float or None, optional
> Threshold for rejecting pixels that deviate below the baseline value.
> If negative value, then will be convert to a positive value.
> If None, no rejection will be done based on low_thresh. Default is 3.
>
> high_thresh : positive float or None, optional
> Threshold for rejecting pixels that deviate above the baseline value.
> If None, no rejection will be done based on high_thresh. Default is 3.

That describes the original ccdproc implementation. In `git show v2.0.1:ccdproc/combiner.py` the method does

```python
if low_thresh is not None:
if low_thresh < 0:
low_thresh = abs(low_thresh)
mask = (self.data_arr - baseline < -low_thresh * dev)
...
if high_thresh is not None:
mask = (self.data_arr - baseline > high_thresh * dev)
...
```

so `None` really did mean "skip that side", and a negative `low_thresh` (only `low_thresh`) was made positive.

Since 2.4.0 (`5d2abc0`, "Always use astropy's sigma clipping and default to string-based functions", #794; the `use_astropy` option itself arrived in 2.2.0 with `d57ca13`) the method forwards the thresholds straight to `astropy.stats.sigma_clip(..., sigma_lower=low_thresh, sigma_upper=high_thresh, ...)`. astropy's `SigmaClip.__init__` (astropy 8.0.1, `astropy/stats/sigma_clipping.py` lines 175–176) does

```python
self.sigma_lower = sigma_lower or sigma
self.sigma_upper = sigma_upper or sigma
```

with `sigma=3` by default, so today:

- `low_thresh=None` / `high_thresh=None` clip at **3σ** on that side rather than not clipping;
- `low_thresh=0` / `high_thresh=0` likewise become 3σ instead of rejecting everything off-centre;
- a negative threshold is passed through unchanged, which turns the comparison inside out (nothing is ever rejected on that side) rather than being converted to a positive value.

None of this is covered by the test suite, and the docstring was not updated in #794.

Related: #1001 (array-API fallback for `sigma_clipping`) deliberately reproduces the astropy semantics on non-NumPy namespaces so all backends agree, and changes the docstring to say that `None` is treated as 3; the negative-threshold wording and the `0` case are left as-is there. #1000 is the companion `sigma_func` change. Both are part of #929.

Read the sigma_clipping docstring in ccdproc/combiner.py around lines 443–452, then inspect its forwarding to astropy.stats.sigma_clip and the related semantics described in the issue. Update the documentation so None, zero, and negative thresholds accurately reflect current behavior, and confirm the documented behavior against the existing test suite.

Written by the indexing model from the issue text.