It's documented along with the CMAKE_INSTALL_PREFIX variable -- since
2.8.10 according to the bug resolution.
http://cmake.org/cmake/help/v2.8.11/cmake.html#variable:CMAKE_INSTALL_PREFIX
If there's a better place for it, perhaps re-open the bug with
additional suggestion. There are plenty of "DESTDIR" references on that
web page, though.
HTH,
D