DOC: `styler.format` does not, in fact, raise a KeyError
This was actually never true from the start #40134. It might have been true at one point during the PR and then was overwritten without docs being corrected before final merge.

I.e. these docs have always been wrong since introduction.

Should be back portable.

```python
df = DataFrame([[1.0, 2.0]], columns=["A", "B"])
df.style.format({"A": "{:.1f}", "B": "{:.2f}"}, subset = ["A"])  
# does not raise a KeyError for "B", just ignores.
```