Add cache variable setting to source tree method

By default both tests, examples and documentation is built when the GLFW
source tree is included in a larger CMake project.

docs/build.dox

@@ -158,7 +158,17 @@ build_link_cmake_module.
 With just a few changes to your `CMakeLists.txt` you can have the GLFW source
 tree built along with your application.
 
-Firstly, add the root directory of the GLFW source tree to your project.  This
+When including GLFW as part of your build, you probably don't want to build the
+GLFW tests, examples and documentation.  To disable these, set the corresponding
+cache variables before adding the GLFW source tree.
+
+@code
+set(GLFW_BUILD_DOCS OFF CACHE BOOL "" FORCE)
+set(GLFW_BUILD_TESTS OFF CACHE BOOL "" FORCE)
+set(GLFW_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)
+@endcode
+
+Then add the root directory of the GLFW source tree to your project.  This
 will add the `glfw` target and the necessary cache variables to your project.
 
 @code{.cmake}