[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/fir/cones/src/org/jetbrains/kotlin/fir/symbols/ConeLookupTags.kt

@@ -9,6 +9,16 @@ import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.types.model.TypeConstructorMarker
 
+/**
+ * The main purpose of the lookup tag is to provide a reference to concrete classifier that
+ * allows obtaining FirClassifierSymbol within a given use-site session.
+ *
+ * During the deserialization we use lookup tags to avoid loading the entire class-type hierarchy.
+ *
+ * Use-site lookup of classifiers that are referenced by lookup tags allows type refinement that is needed for the expect/actual support.
+ *
+ * See `/docs/fir/k2_kmp.md`
+ */
 abstract class ConeClassifierLookupTag : TypeConstructorMarker {
     abstract val name: Name
 
@@ -17,6 +27,9 @@ abstract class ConeClassifierLookupTag : TypeConstructorMarker {
     }
 }
 
+/**
+ * @see ConeClassifierLookupTag
+ */
 abstract class ConeClassLikeLookupTag : ConeClassifierLookupTag() {
     abstract val classId: ClassId