Build: Minor tweaks to GNUInstallDirs defaults It isn't actually necessary to specify `CMAKE_INSTALL_DEFAULT_MANDIR` for our official build. Because `CMAKE_INSTALL_DEFAULT_DATAROOTDIR` is blank for the official build, the default of "<DATAROOTDIR>/man" will resolve to "man". For the same reason, this commit changes the specification of `CMAKE_INSTALL_DEFAULT_DOCDIR` and `CMAKE_INSTALL_DEFAULT_JAVADIR` in the official build to be dependent on the data root directory (mainly to make it obvious what we're doing.) This commit also tweaks the example CMake command line in the directory variable documentation so that it shows the correct location of the CMake argument.