Moved Doxygen comments from source files to public headers.

This commit moves Doxygen comments from source files to the declarations in the public headers so that they are accessible for processing by IDEs and other tools which are using a binary distribution. I did not move the comments for private functions to the private headers because they are accessible to someone developing the    libimobiledevice project itself. If the maintainers would like comments on private functions moved also (for consistency or other motivation), I can do so.