Examples: Comments about GLSL version not being the same as GL context version. (#1828)

examples/opengl3_example/imgui_impl_glfw_gl3.h

@@ -11,6 +11,10 @@
 // If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
 // https://github.com/ocornut/imgui
 
+// About GLSL version:
+// The 'glsl_version' initialization parameter defaults to "#version 150" if NULL.
+// Only override if your GL version doesn't handle this GLSL version. Keep NULL if unsure!
+
 struct GLFWwindow;
 
 IMGUI_API bool        ImGui_ImplGlfwGL3_Init(GLFWwindow* window, bool install_callbacks, const char* glsl_version = NULL);