[Gradle] Document KotlinCompilationTask

Remove KotlinCompilationTask from KotlinCompile class as it anyway comes
via KotlinJvmCompile interface.

^KT-58858 In Progress

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/tasks/KotlinCompilationTask.kt

@@ -10,14 +10,33 @@ import org.gradle.api.Task
 import org.gradle.api.tasks.Nested
 import org.jetbrains.kotlin.gradle.dsl.KotlinCommonCompilerOptions
 
+/**
+ * Represents a Kotlin task compiling using configurable [compilerOptions].
+ *
+ * See [KotlinCommonCompilerOptions] and its inheritors for possible compiler options.
+ *
+ * @see [KotlinCommonCompilerOptions]
+ */
 interface KotlinCompilationTask<out CO : KotlinCommonCompilerOptions> : Task {
+
+    /**
+     * Represents the compiler options used by a Kotlin compilation process.
+     *
+     * This can be used to get the values of currently configured options or modify them.
+     */
     @get:Nested
     val compilerOptions: CO
 
+    /**
+     * Configures the [compilerOptions] with the provided configuration.
+     */
     fun compilerOptions(configure: CO.() -> Unit) {
         configure(compilerOptions)
     }
 
+    /**
+     * Configures the [compilerOptions] with the provided configuration.
+     */
     fun compilerOptions(configure: Action<in CO>) {
         configure.execute(compilerOptions)
     }