http://dlang.org/phobos-prerelease/std_windows_syserror.html
Due to a simple omission, the documentation has not propagated from /phobos-prerelease/ to /phobos/:
Err, to correct/expand on that, the documentation has propagated, it's simply not in the navigation:
http://dlang.org/phobos/std_windows_syserror.html