Initscript, where can I find this method

Help/Discuss
1

I create following script:

initscript {
   repositories {
      mavenCentral()
   }
dependencies {
      classpath 'org.apache.commons:commons-math:2.0'
   }
}

in init.gradle, accorording to gradle documentation:

But I can not find the API for this method.
Where this initscript() comes from?