Add comments for project_build_tools_patch_default_imported_config.

cmake/tools.cmake

@@ -118,6 +118,16 @@ function(project_build_tools_get_imported_location OUTPUT_VAR_NAME TARGET_NAME)
   endif()
 endfunction()
 
+#[[
+If we build third party packages with a different CONFIG setting from building
+otel-cpp, cmake may not find a suitable file in imported targets (#705, #1359)
+when linking. But on some platforms, different CONFIG settings can be used when
+ABI these CONFIG settings have the same ABI. For example, on Linux, we can build
+gRPC and protobuf with -DCMAKE_BUILD_TYPE=Release, but build otel-cpp with
+-DCMAKE_BUILD_TYPE=Debug and links these libraries together.
+The properties of imported targets can be find here:
+https://cmake.org/cmake/help/latest/manual/cmake-properties.7.html#properties-on-targets
+]]
 function(project_build_tools_patch_default_imported_config)
   set(PATCH_VARS
       IMPORTED_IMPLIB