docs: transient_options.md min_dt default is 1e-18 but actual default is 1e-15

**File:** `docs/transient_options.md`, line 91

The 'Internal Parameters' table documents `min_dt` with a default of `1e-18`:

```
| `min_dt` | 1e-18 | Minimum allowed timestep (seconds). |
```

But the actual default in `AdaptiveConfig` is `1e-15`:

```python
# vajax/analysis/transient/full_mna.py
min_dt: float = 1e-15
```

The documented default is three orders of magnitude too small.