[K2] Document KMP implementation in the K2 Compiler

Add comprehensive documentation explaining the implementation and
functions of the Kotlin multiplatform (KMP) support in the K2 Compiler.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/actualizer/IrActualizer.kt

@@ -19,6 +19,13 @@ import org.jetbrains.kotlin.ir.util.*
 
 data class IrActualizedResult(val actualizedExpectDeclarations: List<IrDeclaration>)
 
+/**
+ * IrActualizer is responsible for performing actualization.
+ *
+ * The main action is the replacement of references to expect declarations with corresponding actuals on the IR level.
+ *
+ * See `/docs/fir/k2_kmp.md`
+ */
 object IrActualizer {
     fun actualize(
         mainFragment: IrModuleFragment,