[kotlin/en] Add "if" usage as an expression (#2367)

* [kotlin/en] Add "if" usage as a function

* [kotlin/en] Change comment on "if" expression

Rephrase the comment to use the term "expression" in place of "function"

kotlin.html.markdown

@@ -261,6 +261,14 @@ fun helloWorld(val name : String) {
         ctr++
     } while (ctr < 10)
 
+    /*
+    "if" can be used as an expression that returns a value.
+    For this reason the ternary ?: operator is not needed in Kotlin.
+    */
+    val num = 5
+    val message = if (num % 2 == 0) "even" else "odd"
+    println("$num is $message") // => 5 is odd
+
     // "when" can be used as an alternative to "if-else if" chains.
     val i = 10
     when {