[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/resolution.common/src/org/jetbrains/kotlin/resolve/calls/mpp/AbstractExpectActualCompatibilityChecker.kt

@@ -23,6 +23,12 @@ import org.jetbrains.kotlin.utils.keysToMap
 import org.jetbrains.kotlin.utils.zipIfSizesAreEqual
 import java.util.*
 
+/**
+ * This object is responsible for matching and checking of
+ * expect-actual pairs
+ *
+ * See `/docs/fir/k2_kmp.md` for details
+ */
 object AbstractExpectActualCompatibilityChecker {
     fun <T : DeclarationSymbolMarker> getClassifiersCompatibility(
         expectClassSymbol: RegularClassSymbolMarker,