`JAVA_HOME` should be documented as discriminating

I guess yes, at least for the sake of documenting it.
I see that in `DaemonCompatibilitySpec`, it has a special position among the other discriminating options. But still, it is de facto discriminating, so we should put the discriminating flag on it. WDYT @gnodet ?

Start with DaemonCompatibilitySpec and inspect how JAVA_HOME is represented alongside the other discriminating options. Done means JAVA_HOME is explicitly marked as discriminating and the documented behavior matches that status; the issue does not name a specific test or file beyond this entry point.

Written by the indexing model from the issue text.