Clarify handling of empty arguments in xpath function contains

Author: pijuskri

content/en/docs/refguide/modeling/xpath/xpath-constraints/xpath-constraint-functions/xpath-contains.md

@@ -13,6 +13,28 @@ The `contains()` function tests whether a string attribute contains a specific s
 String comparisons in XPath constraints are generally case-insensitive, but this can depend on the collation setting for some databases. See [Case-Sensitive Database Behavior](/refguide/case-sensitive-database-behavior/) for more information.
 {{% /alert %}}
 
+## Syntax
+
+The syntax is as follows:
+
+```
+contains ( attribute, string_expression )
+```
+
+### attribute
+
+`attribute` specifies the attribute to test in. Attribute must be of the **String** type.
+
+### expression
+
+`string_expression` specifies the value to test for being contained in the attribute. It can be a string literal or an expression that returns a string.
+
+{{% alert color="info" %}}
+In the case the `attribute` is empty or `NULL`, the function will always return `false`, independent of the value of `string_esxpression`.
+
+In the case the `string_expression` is empty, it is treated as an empty string. The function is then equivalent to ```attribute != ''```.
+{{% /alert %}}
+
 ## Example
 
 This query returns all the customers from which the name contains the string `an`: