[FIR tree generator] Add a kDoc for FirUnitExpression

compiler/fir/tree/gen/org/jetbrains/kotlin/fir/expressions/impl/FirUnitExpression.kt

@@ -16,15 +16,19 @@ import org.jetbrains.kotlin.fir.MutableOrEmptyList
 import org.jetbrains.kotlin.fir.builder.toMutableOrEmpty
 import org.jetbrains.kotlin.fir.expressions.FirAnnotation
 import org.jetbrains.kotlin.fir.expressions.FirExpression
+import org.jetbrains.kotlin.fir.expressions.FirReturnExpression
 import org.jetbrains.kotlin.fir.expressions.UnresolvedExpressionTypeAccess
 import org.jetbrains.kotlin.fir.types.ConeKotlinType
 import org.jetbrains.kotlin.fir.types.constructClassLikeType
-import org.jetbrains.kotlin.fir.types.impl.FirImplicitUnitTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.fir.visitors.FirVisitor
 import org.jetbrains.kotlin.fir.visitors.transformInplace
 import org.jetbrains.kotlin.name.StandardClassIds
 
+/**
+ * A special kind of expression that can only appear inside [FirReturnExpression].
+ * It denotes an empty `return` expression, which is different from explicit `return Unit`.
+ */
 class FirUnitExpression @FirImplementationDetail constructor(
     override val source: KtSourceElement?,
     override var annotations: MutableOrEmptyList<FirAnnotation>,

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -371,8 +371,14 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
         }
 
         impl(expression, "FirUnitExpression") {
+            kDoc(
+                """
+                A special kind of expression that can only appear inside [${returnExpression.typeName}].
+                It denotes an empty `return` expression, which is different from explicit `return Unit`.
+                """.trimIndent()
+            )
             defaultBuiltInType("Unit")
-            additionalImports(implicitUnitTypeRefType)
+            additionalImports(returnExpression)
             publicImplementation()
         }