Update data-flow-expression-functions.md

Author: kromerm

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

@@ -70,7 +70,7 @@ In Data Factory and Synapse pipelines, use the expression language of the mappin
 | [hasColumn](data-flow-expressions-usage.md#hasColumn) | Checks for a column value by name in the stream. You can pass an optional stream name as the second argument. Column names known at design time should be addressed just by their name. Computed inputs aren't supported but you can use parameter substitutions.  |
 | [hasError](data-flow-expressions-usage.md#hasError) | Checks if the assert with provided ID is marked as error. |
 | [iif](data-flow-expressions-usage.md#iif) | Based on a condition applies one value or the other. If other is unspecified, it's considered NULL. Both the values must be compatible(numeric, string...).  |
-| [iifNull](data-flow-expressions-usage.md#iifNull) | Checks if the first parameter is null. If not null, the first parameter is returned. If null, the second 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 if the first value isn't null.  |
+| [iifNull](data-flow-expressions-usage.md#iifNull) | Given two or more inputs, returns the first not null item. This function is equivalent to coalesce. |
 | [initCap](data-flow-expressions-usage.md#initCap) | Converts the first letter of every word to uppercase. Words are identified as separated by whitespace.  |
 | [instr](data-flow-expressions-usage.md#instr) | Finds the position(1 based) of the substring within a string. 0 is returned if not found.  |
 | [isDelete](data-flow-expressions-usage.md#isDelete) | Checks if the row is marked for delete. For transformations taking more than one input stream you can pass the (1-based) index of the stream. The stream index should be either 1 or 2 and the default value is 1.  |