Explain the difference betweern 'core_h5p | h5pcustomcss' and 'theme_boost_union | cssh5p'

[abias]

Today, I spotted the 'core_h5p | h5pcustomcss' setting in Moodle Core.
It would be worthwhile to investigate and explain the difference between this setting and 'theme_boost_union | cssh5p' to the admin.

If both settings do practically the same, I would not necessarily remove the settings from Boost Union immediately.
But we should then add a note to the Boost Union setting, similar to our approach with the 'theme_boost_union | loginlocalloginenable' setting