Explain how to concatenate values in Actions mappings

src/connections/destinations/actions.md

@@ -210,6 +210,14 @@ The coalesce function takes a primary value and uses it if it is available. If t
 
 The replace function allows you to replace a string, integer, or boolean with a new value. You have the option to replace up to two values within a single field.
 
+### Concatenate function
+
+To combine two values in the event variable field, you can concatenate them using plain text and variables together. For example, to prepend the country code to a phone number, enter `+1{{Phone Number}}`.
+
+Segment evaluates this field as a string, so placing text next to a variable automatically concatenates them.
+
+![Mapping UI showing two concatenated fields: "+1 phone" and "context.page.url context.page.path"](images/mapping-concatenation.png)
+
 ### Flatten function
 
 The flatten function allows you to flatten a nested object to an object with a depth of 1. Keys are delimited by the configured separator. For example, an object like {a: { b: { c: 1 }, d: 2 } } will be converted to { 'a.b.c': 1, 'a.d': 2 }.
@@ -219,7 +227,6 @@ The flatten function allows you to flatten a nested object to an object with a d
 > info ""
 > Self-service users can add a maximum of two conditions per Trigger.
 
-
 Mapping fields are case-sensitive. The following type filters and operators are available to help you build conditions:
 
 - **Event type** (`is`/`is not`). This allows you to filter by the [event types in the Segment Spec](/docs/connections/spec).