Document String built-in class.

core/builtins/native/kotlin/String.kt

@@ -16,6 +16,10 @@
 
 package kotlin
 
+/**
+ * The `String` class represents character strings. All string literals in Kotlin programs, such as `"abc"`, are
+ * implemented as instances of this class.
+ */
 public class String : Comparable<String>, CharSequence {
     public fun plus(other: Any?): String