[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/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/mpp/FirExpectActualMatcherTransformer.kt

@@ -31,6 +31,14 @@ class FirExpectActualMatcherProcessor(
     }
 }
 
+/**
+ * This transformer populates [expectForActual] mapping for actual declarations.
+ * Also, populates it [memberExpectForActual] mapping
+ *
+ * Should run before any kind of body resolution, since [expectForActual] is used there.
+ *
+ * See `/docs/fir/k2_kmp.md`
+ */
 open class FirExpectActualMatcherTransformer(
     final override val session: FirSession,
     private val scopeSession: ScopeSession,