[FIR] Add KDoc about thread safety rules of FirDeclarationAttributes

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirDeclarationAttributes.kt

@@ -15,6 +15,14 @@ import kotlin.reflect.KProperty
 
 abstract class FirDeclarationDataKey
 
+/**
+ * Please note that FirDeclarationAttributes itself is thread unsafe, so when you read
+ *   or write some attribute you need to ensure that this operation is safe and there
+ *   won't be any race condition. You can achieve this by
+ * - setting attribute to declaration before it's publication (e.g. in scopes)
+ * - setting attribute in one phase and reading it only in following ones (using `ensureResolve` on symbol)
+ * - resetting attribute under lock over specific attribute value
+ */
 class FirDeclarationAttributes : AttributeArrayOwner<FirDeclarationDataKey, Any> {
     override val typeRegistry: TypeRegistry<FirDeclarationDataKey, Any>
         get() = FirDeclarationDataRegistry