FIR: add minor TODO / comments to mismatch checkers

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirInitializerTypeMismatchChecker.kt

@@ -36,9 +36,11 @@ object FirInitializerTypeMismatchChecker : FirPropertyChecker() {
                 expressionType.nullability == ConeNullability.NOT_NULL
             ) {
                 // val p: Byte = 42 or similar situation
+                // TODO: remove after fix of KT-46047
                 return
             }
             if (propertyType.isExtensionFunctionType || expressionType.isExtensionFunctionType) {
+                // TODO: remove after fix of KT-45989
                 return
             }
             reporter.report(INITIALIZER_TYPE_MISMATCH.on(source, propertyType, expressionType), context)

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirFunctionReturnTypeMismatchChecker.kt

@@ -24,6 +24,7 @@ object FirFunctionReturnTypeMismatchChecker : FirReturnExpressionChecker() {
         val targetElement = expression.target.labeledElement
         if (targetElement !is FirSimpleFunction) return
         val resultExpression = expression.result
+        // To avoid duplications with NO_ELSE_IN_WHEN or INVALID_IF_AS_EXPRESSION
         if (resultExpression is FirWhenExpression && !resultExpression.isExhaustive) return
 
         val functionReturnType = targetElement.returnTypeRef.coneType