FindOpenGL: Document how to use macOS X11 OpenGL

Dicourse-topic: https://discourse.cmake.org/t/9466

Modules/FindOpenGL.cmake

@@ -177,9 +177,24 @@ GLVND.  For non-GLVND Linux and other systems these are left undefined.
 macOS-Specific
 ^^^^^^^^^^^^^^
 
-On OSX FindOpenGL defaults to using the framework version of OpenGL. People
-will have to change the cache values of OPENGL_glu_LIBRARY and
-OPENGL_gl_LIBRARY to use OpenGL with X11 on OSX.
+On macOS this module defaults to using the macOS-native framework
+version of OpenGL.  To use the X11 version of OpenGL on macOS, one
+can disable searching of frameworks.  For example:
+
+.. code-block:: cmake
+
+  find_package(X11)
+  if(APPLE AND X11_FOUND)
+    set(CMAKE_FIND_FRAMEWORK NEVER)
+    find_package(OpenGL)
+    unset(CMAKE_FIND_FRAMEWORK)
+  else()
+    find_package(OpenGL)
+  endif()
+
+An end user building this project may need to point CMake at their
+X11 installation, e.g., with ``-DOpenGL_ROOT=/opt/X11``.
+
 #]=======================================================================]
 
 set(_OpenGL_REQUIRED_VARS OPENGL_gl_LIBRARY)