Some warning boxes in the documentation are ugly

[gdementen]

Ugly warnings:
https://larray.readthedocs.io/en/stable/tutorial/tutorial_sessions.html#Selecting-And-Filtering-Items

It must be possible to do it nicely, because in other sections they are nice, for example in:
https://larray.readthedocs.io/en/stable/tutorial/tutorial_presenting_larray_objects.html#Axis

[gdementen]

done (probably by #1047)