Backends: Comments next to include, misc minor comments/tweaks, fix imgui_impl_osx.h using IMGUI_API instead of IMGUI_IMPL_API. (#3105)

examples/imgui_impl_opengl3.h

@@ -22,8 +22,7 @@
 //  Only override if your GL version doesn't handle this GLSL version. See GLSL version table at the top of imgui_impl_opengl3.cpp.
 
 #pragma once
-
-#include "imgui.h"
+#include "imgui.h"      // IMGUI_IMPL_API
 
 // Backend API
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_Init(const char* glsl_version = NULL);