Backends: OpenGL: Update comments. (#4445, #5642, #4649)

backends/imgui_impl_opengl3_loader.h

@@ -9,6 +9,14 @@
 // YOU SHOULD NOT NEED TO INCLUDE/USE THIS DIRECTLY. THIS IS USED BY imgui_impl_opengl3.cpp ONLY.
 // THE REST OF YOUR APP SHOULD USE A DIFFERENT GL LOADER: ANY GL LOADER OF YOUR CHOICE.
 //
+// IF YOU GET BUILD ERRORS IN THIS FILE (commonly macro redefinitions or function redefinitions):
+// IT LIKELY MEANS THAT YOU ARE BUILDING 'imgui_impl_opengl3.cpp' OR INCUDING 'imgui_impl_opengl3_loader.h'
+// IN THE SAME COMPILATION UNIT AS ONE OF YOUR FILE WHICH IS USING A THIRD-PARTY OPENGL LOADER.
+// (e.g. COULD HAPPEN IF YOU ARE DOING A UNITY/JUMBO BUILD, OR INCLUDING .CPP FILES FROM OTHERS)
+// YOU SHOULD NOT BUILD BOTH IN THE SAME COMPILATION UNIT.
+// BUT IF YOU REALLY WANT TO, you can '#define IMGUI_IMPL_OPENGL_LOADER_CUSTOM' and imgui_impl_opengl3.cpp
+// WILL NOT BE USING OUR LOADER, AND INSTEAD EXPECT ANOTHER/YOUR LOADER TO BE AVAILABLE IN THE COMPILATION UNIT.
+//
 // Regenerate with:
 //   python gl3w_gen.py --output ../imgui/backends/imgui_impl_opengl3_loader.h --ref ../imgui/backends/imgui_impl_opengl3.cpp ./extra_symbols.txt
 //