Correct wx/msw/setup.h instructions in the documentation

This file doesn't need to be created initially, as it will be done
automatically during the build, but it does need to be updated whenever
setup0.h changes, so explain this in both the README-GIT.md file and the
main build instructions.

Closes #18699.

docs/msw/install.md

@@ -34,16 +34,21 @@ variable containing the full path to this directory. While this is not
 actually required, this makes using the library more convenient and
 this environment variable is used in the examples below.
 
-NB: If you checked your sources from version control repository and
-didn't obtain them from a release file, you also need to copy
-`include/wx/msw/setup0.h` to `include/wx/msw/setup.h` and to remember
-to update the latter whenever the former changes, otherwise you
-will get compilation errors if any new symbols are added to
-setup0.h file in the repository.
+Upgrading Existing Git Checkout         {#msw_update_setup_h}
+-------------------------------
 
-If you have no intention of modifying setup.h, you may avoid this
-problem by creating a symbolic link to setup0.h instead of making
-a copy of it using mklink, from an admin command prompt:
+If you checked your sources from version control repository and didn't
+obtain them from a release file, you may need to update your
+`include/wx/msw/setup.h` file to add any new options from
+`include/wx/msw/setup0.h` to it. If you hadn't edited `setup.h` file
+manually, you can just delete it to force recreating it by copying
+`setup0.h` to `setup.h` during the next build. If you forget to do it,
+you may get errors during the build due to new options being
+undefined.
+
+If you have no intention of ever modifying `setup.h`, you may avoid this
+problem entirely by creating a symbolic link to `setup0.h` before
+building, e.g. using mklink, from an admin command prompt:
 
     cd %WXWIN%\include\wx\msw\
     mklink setup.h setup0.h