gradle dependencies: what "classpath" keyword means exactly and where it is documented?

I've did web search for "gradle classpath site:stackoverflow.com" and "gradle classpath" and found relevant info in only:
Gradle: What is the difference between classpath and compile dependencies?

In the answer by Teng-pao Yu it is written:

compile 'org.hibernate:hibernate-core:5.0.5.Final' is a module dependency declaration. The compile configuration (which is now deprecated by the implementation configuration.) is merely a keyword for Implementation only dependencies. It is not a keyword describing which type of dependency it is (by type here I'm following the three types defined in the tutorial, i.e. module, file, and project.)

So as I understood classpath is also keyword. I've tried to find its` meaning in gradle docs: https://docs.gradle.org/current/userguide/declaring_dependencies.html

And some others referenced in it:
https://docs.gradle.org/current/userguide/dependency_management_for_java_projects.html https://docs.gradle.org/current/userguide/java_library_plugin.html https://docs.gradle.org/current/dsl/org.gradle.api.artifacts.Configuration.html https://docs.gradle.org/current/userguide/variant_model.html https://docs.gradle.org/current/userguide/java_plugin.html

Also: https://docs.gradle.org/current/javadoc/org/gradle/api/initialization/dsl/ScriptHandler.html https://docs.gradle.org/current/javadoc/org/gradle/api/tasks/Classpath.html https://docs.gradle.org/current/userguide/organizing_gradle_projects.html

There are mentions of 'compileClasspath'. If classpath keyword is merely deprecated as compile one, why is it absent from docs?

P.S. I mean like in:

buildscript {
    repositories {
        jcenter()
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.1'
   }
}

ADDED after answer:

search for getBuildscript() in gradle source code: