[Gradle] Add documentation for LanguageSettings

^KT-58858 In Progress

libraries/tools/kotlin-gradle-plugin-api/src/common/kotlin/org/jetbrains/kotlin/gradle/dsl/LanguageSettings.kt

@@ -7,10 +7,55 @@
 
 package org.jetbrains.kotlin.project.model
 
+/**
+ * Represents most common Kotlin compilation settings for an entity.
+ *
+ * **Note**: This interface is soft-deprecated.
+ * Instead, better to use existing `compilerOptions` DSL.
+ *
+ * See also [Compiler options DSL documentation](https://kotlinlang.org/docs/gradle-compiler-options.html).
+ */
 interface LanguageSettings {
+
+    /**
+     * Provides source compatibility with the specified version of Kotlin.
+     *
+     * Possible values: "1.4 (deprecated)", "1.5 (deprecated)", "1.6", "1.7", "1.8", "1.9", "2.0 (experimental)", "2.1 (experimental)".
+     *
+     * Default value: `null`
+     */
     val languageVersion: String?
+
+    /**
+     * Allows using declarations only from the specified version of bundled libraries.
+     *
+     * Possible values: "1.4 (deprecated)", "1.5 (deprecated)", "1.6", "1.7", "1.8", "1.9", "2.0 (experimental)", "2.1 (experimental)".
+     *
+     * Default value: `null`
+     */
     val apiVersion: String?
+
+    /**
+     * Enables progressive compiler mode.
+     *
+     * In this mode, deprecations and bug fixes for unstable code take effect immediately,
+     * instead of going through a graceful migration cycle.
+     * Code written in progressive mode is backward compatible. However, code written in
+     * non-progressive mode may cause compilation errors in progressive mode.
+     *
+     *  Default value: false
+     */
     val progressiveMode: Boolean
+
+    /**
+     * @suppress
+     */
     val enabledLanguageFeatures: Set<String>
+
+    /**
+     * Enables use of any API that requires opt-in with an opt-in requirement marker containing its fully qualified name.
+     *
+     * Default value: emptyList<String>()
+     */
     val optInAnnotationsInUse: Set<String>
-}
+}