fixing iifNull

Author: djpmsft

articles/data-factory/data-flow-expression-functions.md

@@ -320,10 +320,11 @@ Based on a condition applies one value or the other. If other is unspecified it
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 ### <code>iifNull</code>
 <code><b>iifNull(<i>&lt;value1&gt;</i> : any, [<i>&lt;value2&gt;</i> : any], ...) => any</b></code><br/><br/>
-Checks if the value is NOT NULL and returns it else returns the alternate. It tests for all inputs until it finds the first non-null value  
+Checks if the first parameter is null. If not null, the value is returned. If null, the parameter is returned. If three parameters are specified, the behavior is the same as iif(isNull(value1), value2, value3) and the third parameter is returned.  
 * ``iifNull(10, 20) -> 10``  
 * ``iifNull(null, 20, 40) -> 20``  
-* ``iifNull('bojjus', 'bo', 'dumbo') -> 'dumbo'``  
+* ``iifNull('azure', 'data', 'factory') -> 'factory'``  
+* ``iifNull(null, 'data', 'factory') -> 'data'``  
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 ### <code>in</code>
 <code><b>in(<i>&lt;array of items&gt;</i> : array, <i>&lt;item to find&gt;</i> : any) => boolean</b></code><br/><br/>