I am not one of the authors, but obviously they thought this style is more readable than putting everything on a single line. The perceived advantage of this style is that you can easily find the name, qualifiers and parameters of each function, because they are always at the same position (indent depth).
Moreover, when using code documenting tools such as Doxygen, this style allows one to append parameter doc comments on the same line right after each parameter, which (IMHO) makes them easier to read and maintain.
Even with the single line approach, often you need to break lines when they get too long. And in WinAPI files, looong function declarations are pretty common :-) So probably at some point they decided to have a "standard" approach as to where to break those lines.