receiver section update

Author: sarahhaggarty

docs/topics/extensions.md

@@ -14,29 +14,26 @@ only making new functions callable or new properties accessible using special sy
 Extensions are always called on a receiver. The receiver has to have the same type as the class or interface being extended.
 To use an extension, prefix it with the receiver followed by a `.` and the function or property name.
 
-For example, the [`.orEmpty()`](https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/or-empty.html) extension function
-from the standard library extends the `String?` class:
+For example, the [`.appendLine()`](https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/or-empty.html) extension function from the standard library extends the `StringBuilder` class.
+So in this case, the receiver is a `StringBuilder` instance, and the _receiver type_ is `StringBuilder`:
 
 ```kotlin
-fun String?.orEmpty(): String
-```
-
-So in this case, the receiver is a `String?` instance, and the _receiver type_ is `String?`:
-
-```kotlin
-fun main() {
+fun main() { 
 //sampleStart
-    // nullableString is an instance of String?
-    val nullableString: String? = null
-    // Calls .orEmpty() extension function on nullableString
-    val nonNullString = nullableString.orEmpty()
-
-    println("Is the string empty? ${nonNullString == ""}") 
-    // Is the string empty? true
-//sampleEnd
+    // builder is an instance of StringBuilder
+    val builder = StringBuilder()
+        // Calls .appendLine() extension function on builder
+        .appendLine("Hello")
+        .appendLine()
+        .appendLine("World")
+    println(builder.toString())
+    // Hello
+    //
+    // World
 }
+//sampleEnd
 ```
-{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-extension-function-isorempty"}
+{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-extension-function-stringbuilder"}
 
 ## Extension functions
 
@@ -123,8 +120,8 @@ returns a non-null value.
 ### Generic extension functions
 
 To create generic extension functions, declare the generic type parameter before the function name
-to make it available in the receiver type expression. In this example, the `.endpoints()` function extends a `List<T>` 
-class where `T` can be any type:
+to make it available in the receiver type expression. In this example, the `.endpoints()` function extends `List<T>` 
+where `T` can be any type:
 
 ```kotlin
 fun <T> List<T>.endpoints(): Pair<T, T> {