Fix Macro readthedocs

[shroffk]

I believe the documentation is incorrect here

https://control-system-studio.readthedocs.io/en/latest/app/display/editor/doc/macros.html#system-macros

The macros cannot be overwritten by symbol macros

[shroffk]

@kasemir I can go ahead and push a fix but wanted your review to verify my understanding

[kasemir]

That would be perfect, thanks!