[DOCS] Update painless statements with if/else example (#47485)

Author: renshuki

docs/painless/painless-statements.asciidoc

@@ -4,11 +4,28 @@
 Painless supports all of Java's https://docs.oracle.com/javase/tutorial/java/nutsandbolts/flow.html[
 control flow statements] except the `switch` statement.
 
+==== Conditional statements
+
+===== If / Else
+
+[source,painless]
+---------------------------------------------------------
+if (doc[item].size() == 0) {
+  // do something if "item" is missing
+} else {
+  // do something else
+}
+---------------------------------------------------------
+
+==== Loop statements
+
+===== For
+
 Painless also supports the `for in` syntax from Groovy:
 
 [source,painless]
 ---------------------------------------------------------
 for (item : list) {
   ...
 }
----------------------------------------------------------
+---------------------------------------------------------