Correct docs of ThreadLocal and SharedImmutable annotations

- clarify that they have effect only in K/N
- correct possible application targets of SharedImmutable

KT-36245

runtime/src/main/kotlin/kotlin/native/concurrent/Annotations.kt

@@ -6,11 +6,13 @@
 package kotlin.native.concurrent
 
 /**
- * Marks a top level variable with a backing field or an object as thread local.
+ * Marks a top level property with a backing field or an object as thread local.
  * The object remains mutable and it is possible to change its state,
  * but every thread will have a distinct copy of this object,
  * so changes in one thread are not reflected in another.
  *
+ * The annotation has effect only in Kotlin/Native platform.
+ *
  * PLEASE NOTE THAT THIS ANNOTATION MAY GO AWAY IN UPCOMING RELEASES.
  */
 @Target(AnnotationTarget.PROPERTY, AnnotationTarget.CLASS)
@@ -18,10 +20,12 @@ package kotlin.native.concurrent
 public actual annotation class ThreadLocal
 
 /**
- * Marks a top level variable with a backing field or an object as immutable.
- * It is possible to share such object between multiple threads, but it becomes deeply frozen,
+ * Marks a top level property with a backing field as immutable.
+ * It is possible to share the value of such property between multiple threads, but it becomes deeply frozen,
  * so no changes can be made to its state or the state of objects it refers to.
  *
+ * The annotation has effect only in Kotlin/Native platform.
+ *
  * PLEASE NOTE THAT THIS ANNOTATION MAY GO AWAY IN UPCOMING RELEASES.
  */
 @Target(AnnotationTarget.PROPERTY)