This is something Kotlin DSL specific, as Groovy DSL script are duck-typed anyway.
Thus, it is documented on the Kotlin DSL documentation page: Gradle Kotlin DSL Primer
Also, it is not really about convention plugins, but about precompiled script plugins.
Convention plugins can be developed in any JVM language and as precompiled script plugins, binary plugins, or whatever.
But it is also not mentioned on the precompiled script plugins page.
As Kotlin DSL is now promoted as default DSL, I agree that at least the precompiled script plugins page should maybe mention it.
I’d recommend you open a feature request issue or PR to improve the docs. 