The //> using exclude directive is silently ignored when declared in any file other than project.scala

[Gedochao]

Version(s)
1.5.4 and 4ab3227

Describe the bug
If the //> using exclude (..) directive is declared anywhere but project.scala, it gets ignored completely, with no warning.
While we do not want it declared in other files (and that is intended), we should communicate it better in our docs as well as actually warn users about it being ignored.
The current behaviour is very confusing.

To Reproduce
Try to use the //> using exclude directive in any file other than project.scala.

Expected behaviour

• clear explanation in the docs
• a proper warning when //> using exclude is declared wrongly