Comments/fixes about using IMGUI_DEFINE_MATH_OPERATORS in imconfig.h (#9524)

diff --git a/imconfig.h b/imconfig.h
index b40db38..fd1ed81 100644
--- a/imconfig.h
+++ b/imconfig.h

@@ -114,8 +114,11 @@
         constexpr ImVec4(const MyVec4& f) : x(f.x), y(f.y), z(f.z), w(f.w) {}   \
         operator MyVec4() const { return MyVec4(x,y,z,w); }
 */
-//---- ...Or use Dear ImGui's own very basic math operators.
+//---- ...Or use Dear ImGui's own basic math operators in every file including imgui.h:
+//#ifndef IMGUI_DEFINE_MATH_OPERATORS
 //#define IMGUI_DEFINE_MATH_OPERATORS
+//#endif
+//---- ...Or manually '#define IMGUI_DEFINE_MATH_OPERATORS' before '#include "imgui.h"' to use from one file.
 
 //---- Use 32-bit vertex indices (default is 16-bit) is one way to allow large meshes with more than 64K vertices.
 // Your renderer backend will need to support it (most example renderer backends support both 16/32-bit indices).

diff --git a/misc/single_file/imgui_single_file.h b/misc/single_file/imgui_single_file.h
index 7ca31e0..bea1cb6 100644
--- a/misc/single_file/imgui_single_file.h
+++ b/misc/single_file/imgui_single_file.h

@@ -1,14 +1,14 @@
 // dear imgui: single-file wrapper include
 // We use this to validate compiling all *.cpp files in a same compilation unit.
-// Users of that technique (also called "Unity builds") can generally provide this themselves,
-// so we don't really recommend you use this in your projects.
+// Users of that technique (also called "Unity builds") can generally provide this themselves easily.
 
 // Do this:
 //    #define IMGUI_IMPLEMENTATION
-// Before you include this file in *one* C++ file to create the implementation.
+//    #include "imgui_single_file.h"
+// In *one* C++ file to create the implementation.
 // Using this in your project will leak the contents of imgui_internal.h and ImVec2 operators in this compilation unit.
 
-#ifdef IMGUI_IMPLEMENTATION
+#if defined(IMGUI_IMPLEMENTATION) && !defined(IMGUI_DEFINE_MATH_OPERATORS)
 #define IMGUI_DEFINE_MATH_OPERATORS
 #endif