Add markers at semantic definition end

[atomczak]

As discussed here: https://github.com/IfcOpenShell/IfcOpenShell/issues/4624, it's worth indicating the end of the short semantic definition and the beginning of the usually long and technical description. This marker, apart from enabling software to read short definitions, will be used to trim the definitions ending up in the bSDD.

To not disrupt the documentation, let's use as a marker an HTML comment: <!-- end of definition -->.