Various improvements in collection samples #KT-20357

Merge some related samples,
add sample for building a sequence from Enumeration similar to building a list from Enumeration.

libraries/stdlib/src/kotlin/collections/Collections.kt

@@ -132,7 +132,7 @@ public fun <T> arrayListOf(vararg elements: T): ArrayList<T>
 
 /**
  * Returns a new read-only list either of single given element, if it is not null, or empty list if the element is null. The returned list is serializable (JVM).
- * @sample samples.collections.Collections.Lists.singletonListOfNotNull
+ * @sample samples.collections.Collections.Lists.listOfNotNull
  */
 public fun <T : Any> listOfNotNull(element: T?): List<T> = if (element != null) listOf(element) else emptyList()
 

libraries/stdlib/src/kotlin/collections/Sequences.kt

@@ -24,6 +24,7 @@ public fun <T> Iterator<T>.asSequence(): Sequence<T> = Sequence { this }.constra
 
 /**
  * Creates a sequence that returns all values from this enumeration. The sequence is constrained to be iterated only once.
+ * @sample samples.collections.Sequences.Building.sequenceFromEnumeration
  */
 @kotlin.jvm.JvmVersion
 @kotlin.internal.InlineOnly