Add comments to skipped FIR tests

compiler/testData/codegen/box/builtinStubMethods/mapRemove/readOnlyMap.kt

@@ -1,3 +1,4 @@
+// TODO: FirModuleDescriptor provides default builtins which filters out platform-specific functions (and defaults are considered platform-specific)
 // IGNORE_BACKEND_FIR: JVM_IR
 // SKIP_JDK6
 // TARGET_BACKEND: JVM

compiler/testData/codegen/bytecodeText/constProperty/noInline.kt

@@ -1,6 +1,8 @@
 // !LANGUAGE: -InlineConstVals
 // IGNORE_BACKEND_FIR: JVM_IR
 
+// Fir2Ir IrConstTransformer inline everything it can unconditionally
+// No fix atm, we don't have to support this feature
 const val z = 0
 
 fun a() {