Where should I put documentation comments?

As far as I know, every time you change something in the .h file, it causes all files that have included that header file to be recompiled. For this reason, I put most of my comments in the .cpp files.

For C++, I put "documentation comments" in both the cpp and h.

The cpp contains the code, and has documentation comments on every main code element (classes, methods, etc) that describe them - typically with doxygen or Documentation XML comment format (although I don't tend to generate external docs, I find it useful to stick to a standardised format that can be extracted to external tools if/when I decide I want that). This is comprehensive documentation that explains exactly how a caller should use a method, and also any design details that will need to be understood by anyone intending to modify the code, so they understand the intent, "contract", and any important things to understand about the operation of the code. (I've written an addin for Visual Studio, AtomineerUtils, that makes creating and updating these comments quick and easy, so it's really not much effort to document things like this consistently and comprehensively)

My header is treated as a summary (both for the compiler and myself) - it uses a single-line // comment that briefly describes each method. This gives more information than the (hopefully relatively self-documenting) method/parameter names, but a lot less than the detailed documentation in the cpp. Think of it as a summary or reminder that saves you looking at the actual implementation to get enough details to use the method most of the time.

For usage information, it's nicer to put into the header. That's where people would look first.

The documentation is really successful if no one has to examine your .cpp file to figure out how the component should be used.