NIFI-1826 Updated documentation

Signed-off-by: Matt Burgess <mattyb149@apache.org>

This closes #396

nifi-docs/src/main/asciidoc/expression-language-guide.adoc

@@ -1035,6 +1035,28 @@ Each of the following functions is used to search its subject for some value.
 
 
 
+[.function]
+=== in
+
+*Description*: [.description]#Returns `true` if the Subject is matching one of the provided arguments.#
+
+*Subject Type*: [.subject]#String#
+
+*Arguments*:
+
+	- [.argName]#_value1_# : [.argDesc]#First possible matching value#
+	- [.argName]#_valueN_# : [.argDesc]#Nth possible matching value#
+
+*Return Type*: [.returnType]#Boolean#
+
+*Examples*: If the "myEnum" attribute has the value "JOHN", then the Expression
+	`${myEnum:in("PAUL", "JOHN", "MIKE")}` will return `true`. `${myEnum:in("RED", "GREEN", "BLUE")}` will
+	return `false`.
+
+
+
+
+
 [.function]
 === find