You're downvoted for some mysterious reason, but you're completely right, in languages with module support I really miss the compact API overview that is provided by C headers. This is good because the interface is important, and the implementation is not.

In modules, the implementation completely dominates, and it's impossible to figure out what the module API looks like from looking at the source. You need an IDE to generate an outline for you, which frankly sucks.

For C at least, headers are pretty much perfect because they are supposed to only contain public API declarations. It's just C++ that messed everything up with the inline and template keywords, which required implementation code to move into header files.