Fix CoroutineContext documentation with respect to equality

KT-26398 had updated behavior from reference comparison to regular
equality, but the documentation for CoroutineContext was left out.
The whole "by reference" needs to be removed, because it now
follows default behavior of all Kotlin collections and need no
further clarification.

libraries/stdlib/src/kotlin/coroutines/CoroutineContext.kt

@@ -8,14 +8,12 @@ package kotlin.coroutines
 /**
  * Persistent context for the coroutine. It is an indexed set of [Element] instances.
  * An indexed set is a mix between a set and a map.
- * Every element in this set has a unique [Key]. Keys are compared _by reference_.
+ * Every element in this set has a unique [Key].
  */
 @SinceKotlin("1.3")
 public interface CoroutineContext {
     /**
      * Returns the element with the given [key] from this context or `null`.
-     * Keys are compared _by reference_, that is to get an element from the context the reference to its actual key
-     * object must be presented to this function.
      */
     public operator fun <E : Element> get(key: Key<E>): E?
 
@@ -46,14 +44,12 @@ public interface CoroutineContext {
 
     /**
      * Returns a context containing elements from this context, but without an element with
-     * the specified [key]. Keys are compared _by reference_, that is to remove an element from the context
-     * the reference to its actual key object must be presented to this function.
+     * the specified [key].
      */
     public fun minusKey(key: Key<*>): CoroutineContext
 
     /**
      * Key for the elements of [CoroutineContext]. [E] is a type of element with this key.
-     * Keys in the context are compared _by reference_.
      */
     public interface Key<E : Element>