How to add sources and javadoc from an external library with gradle and IntelliJ?

I have a Java project that I am developing with IntelliJ and Gradle. I have defined the idea block in me build.gradlethat, in short, what it does is download the sources and javadoc of those libraries that are in some maven repository:

idea{
    module {
        downloadJavadoc = true // defaults to false
        downloadSources = true
    }
}

So far everything is correct. However, I have an external library, which is not found in any maven repository. To import this library I have defined, in the repositories block , the libs folder, in which I have dropped the library and its corresponding javadoc.

repositories {
    // repositorios maven
    flatDir {
        dirs 'libs'
    }
}

I have added the corresponding dependencies in the dependencies block . The project compiles correctly and loads the javadoc of those libraries that are downloaded from a maven repository, however it does not load the javadoc of the external library in question. Do I have to somehow define the javadoc of said library in my build.gradle?

Thanks in advance.