[REQ] [DOCUMENTATION] outputDir is required when generating multiple clients (Java, Gradle)

The problem

I was adding a second Java client to a gradle project with 1 client. The existing gradle config was like this (for anonymity's sake, I'm building hello world as a service).

openApiGenerate {
    validateSpec = true
    inputSpec = "$rootDir/openapi/hello_openapi.yaml"
    generatorName = "java"
    library = "resttemplate"
    modelPackage = "com.hello_world_client.generated.model"
    apiPackage = "com.hello_world_client.generated.api"
    invokerPackage = "com.hello_world_client.generated"
    configOptions = [
            dateLibrary: "java8"
    ]
}

Changing it to this results in a very cryptic error about a './docs/HelloTemplate.md' not being found.

task buildHelloClient(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTask) {
   // identical config
}

The error is resolved by adding outputDir to the config (which I only tried because it's used in the examples). I didn't lose too much time, but still annoying.

Describe the solution you'd like

I'd like the section on generating multiple sources to explicitly state that outputDir is required.

Describe alternatives you've considered

I proposed the lowest effort solution since this isn't a big deal really, but to solve the issue itself, some alternatives:

• the generator be improved that any configuration in a openApiGenerate block is works with a task that extends GenerateTask and vice versa
• the generator may try to detect this situation and throw a clear exception.

Start with the documentation section and examples covering generation of multiple sources and the Gradle GenerateTask. Document that outputDir is required when generating multiple clients, then verify the example configuration and wording against the reported Gradle behavior.