Hi Emmanuelle,
I have to agree with OP, that the documentation for plotly is lacking. Considering this exact issue, I was dumbfounded that the documentation for hover events for plotly.js is just a tiny portion of what the same documentation for python is.
I.e. https://plotly.com/python/hover-text-and-formatting/#customizing-hover-text-with-a-hovertemplate
vs
https://plotly.com/javascript/hover-text-and-formatting/
On the js documentation page, searching for “|” results in … nothing. It doesn’t mention it at all. It is not very logical to start looking at the other docs for python, R, dash in the hope that more info can be found there.