Repeat target specific information in "target details".
Users don't read entire manuals and target specific details embedded elsewhere should at least be cross-referenced in each target's subsection in chapter 12. This is a good place to elaborate on differences in compilation switches, for example.
I agree with this, I did find the target-specific sections a bit too thin. Do you have any specific suggestions apart from the compiler defines?
After looking a the other targets, it appears that Java is the only one that doesn't include information about compiler switches. You could also include a link to the language's API section.
The compiler table currently doesn't link to the language section.
Everything else I would say about targets I know about belong in the comments section.