[docs] updated the part about Boost in installation guide (#2082)

* updated installation guide

* updated Python installation guide

* added note about opencl path to Windows section

* added space before path in message

* minor correction for option description in Python installation guide

CMakeLists.txt

@@ -93,7 +93,7 @@ if(USE_GPU)
     include_directories(${BOOST_COMPUTE_HEADER_DIR})
     find_package(OpenCL REQUIRED)
     include_directories(${OpenCL_INCLUDE_DIRS})
-    MESSAGE(STATUS "OpenCL include directory:" ${OpenCL_INCLUDE_DIRS})
+    MESSAGE(STATUS "OpenCL include directory: " ${OpenCL_INCLUDE_DIRS})
     if (WIN32)
         set(Boost_USE_STATIC_LIBS ON)
     endif()

docs/Installation-Guide.rst

@@ -563,16 +563,16 @@ Following procedure is for the **MSVC** (Microsoft Visual C++) build.
 
    .. code::
 
-     Set BOOST_ROOT=C:\local\boost_1_63_0\
-     Set BOOST_LIBRARYDIR=C:\local\boost_1_63_0\lib64-msvc-14.0
      git clone --recursive https://github.com/Microsoft/LightGBM
      cd LightGBM
      mkdir build
      cd build
-     cmake -A x64 -DUSE_GPU=1 ..
+     cmake -A x64 -DUSE_GPU=1 -DBOOST_ROOT=C:/local/boost_1_63_0 -DBOOST_LIBRARYDIR=C:/local/boost_1_63_0/lib64-msvc-14.0 ..
+     # if you have installed NVIDIA CUDA to a customized location, you should specify paths to OpenCL headers and library like the following:
+     # cmake -A x64 -DUSE_GPU=1 -DBOOST_ROOT=C:/local/boost_1_63_0 -DBOOST_LIBRARYDIR=C:/local/boost_1_63_0/lib64-msvc-14.0 -DOpenCL_LIBRARY="C:/Program Files/NVIDIA GPU Computing Toolkit/CUDA/v10.0/lib/x64/OpenCL.lib" -DOpenCL_INCLUDE_DIR="C:/Program Files/NVIDIA GPU Computing Toolkit/CUDA/v10.0/include" ..
      cmake --build . --target ALL_BUILD --config Release
 
-   **Note**: ``C:\local\boost_1_63_0\`` and ``C:\local\boost_1_63_0\lib64-msvc-14.0`` are locations of your **Boost** binaries (assuming you've downloaded 1.63.0 version). You also can set them to the environment variable to avoid ``Set ...`` commands when build.
+   **Note**: ``C:/local/boost_1_63_0`` and ``C:/local/boost_1_63_0/lib64-msvc-14.0`` are locations of your **Boost** binaries (assuming you've downloaded 1.63.0 version for Visual Studio 2015).
 
 Docker
 ^^^^^^

python-package/README.rst

@@ -92,7 +92,7 @@ All remarks from `Build from Sources section <#build-from-sources>`__ are actual
 
 For **Windows** users, `CMake`_ (version 3.8 or higher) is strongly required.
 
-**Boost** and **OpenCL** are needed: details for installation can be found in `Installation Guide <https://github.com/Microsoft/LightGBM/blob/master/docs/Installation-Guide.rst#build-gpu-version>`__. You need to add ``OpenCL_INCLUDE_DIR`` to the environmental variable **'PATH'** and export ``BOOST_ROOT`` before installation. Alternatively, you may pass options to **CMake** via ``pip`` options, like
+**Boost** and **OpenCL** are needed: details for installation can be found in `Installation Guide <https://github.com/Microsoft/LightGBM/blob/master/docs/Installation-Guide.rst#build-gpu-version>`__. Almost always you also need to pass ``OpenCL_INCLUDE_DIR``, ``OpenCL_LIBRARY`` options for **Linux** and ``BOOST_ROOT``, ``BOOST_LIBRARYDIR`` options for **Windows** to **CMake** via ``pip`` options, like
 
 .. code:: sh