Missing documentation that PATH has to contain path to libllvm.dll to build Qt documentation from source

On page

https://doc.qt.io/qt-5/qdoc-guide-clang.html#specify-clang-location-manually

there is a hint for windows, that the environment variable LLVM_INSTALL_DIR has to point to the install directory of LLVM.

An this seems to be sufficient for the configure of the Qt building process along with the testing if Qt documentation can be build.

But actually setting the PATH variable to also contain the "bin" directory of the LLVM installation is critical to let the building of the Qt documentation work.

Some text like the following could be added:

For Windows also add the folder for binaries and dlls to the PATH environment variable. e.g.

C:\> set PATH=%LLVM_INSTALL_DIR%\bin;%PATH%