Add note in docs on score_weight config if using a non-default spans_key for SpanCat (#13093)

* Add note on score_weight if using a non-default span_key for SpanCat.

* Fix formatting.

* Fix formatting.

* Fix typo.

* Use warning infobox.

* Fix infobox formatting.

website/docs/api/spancategorizer.mdx

@@ -89,6 +89,20 @@ architectures and their arguments and hyperparameters.
 | `negative_weight` <Tag variant="new">3.5.1</Tag>    | Multiplier for the loss terms. It can be used to downweight the negative samples if there are too many. It is only used when `add_negative_label` is `True`. Defaults to `1.0`. ~~float~~                                                                                                               |
 | `allow_overlap` <Tag variant="new">3.5.1</Tag>      | If `True`, the data is assumed to contain overlapping spans. It is only available when `max_positive` is exactly 1. Defaults to `True`. ~~bool~~                                                                                                                                                        |
 
+<Infobox variant="warning">
+
+If you set a non-default value for `spans_key`, you'll have to update
+`[training.score_weights]` as well so that weights are computed properly. E. g. for `span_key == "myspankey"`, include this in your config:
+
+```ini
+[training.score_weights]
+spans_myspankey_f = 1.0
+spans_myspankey_p = 0.0
+spans_myspankey_r = 0.0
+```
+
+</Infobox>
+
 ```python
 %%GITHUB_SPACY/spacy/pipeline/spancat.py
 ```