Add note of separate compilation unit in macro's code example

[mosyp]

Old docs (https://docs.scala-lang.org/overviews/macros/overview.html#a-complete-example) has a paragraph which says:

An important aspect of macrology is separate compilation. To perform macro expansion, compiler needs a macro implementation in executable form. Thus macro implementations need to be compiled before the main compilation, otherwise you might see the following error:

However dotty throws unclear error so far so I opened and issue #4540 and submitted only simple note to code snippet as a remainder

[dottybot]

Hello, and thank you for opening this PR! 🎉

All contributors have signed the CLA, thank you! ❤️

Have an awesome day! ☀️

[nicolasstucki]

We can be even clearer.

}
object App {
   // has to be in a different compilation unit that depends on Macros

[mosyp]

make sense!

[nicolasstucki]

@mentegy did you sigh the CLA?

[allanrenucci]

CLA signed

[mosyp]

@nicolasstucki done

[nicolasstucki]

Thanks @mentegy