1. install the Jenkins Parameterized Trigger plugin (2.35.2)
2. create a freestyle job
3. click Add build step
4. click Trigger/call builds on [sic] other projects
5. click Add ParameterFactories [sic]
6. click For every matching file, invoke one build
7. click  next to File Name In Child Projects
8. See a blank help box.

Expected results: there should be help.

The relevant empty file is:

https://github.com/jenkinsci/parameterized-trigger-plugin/blob/55f8d5b10f00164a090972f8af89794acddf870b/src/main/resources/hudson/plugins/parameterizedtrigger/matrix/MatrixSubsetBuildParameters/help-filter.html#L20

I'm not going to fix this right away. Filing it because I found it while reading the help for everything and I'm punting as I'm not quite sure about the fix.