Add explanation for Device ID coalesce

Added explanation for the `coalesce` function in the Device ID field, to clear up customer confusion.

Author: jenskene

src/connections/destinations/catalog/actions-amplitude/index.md

@@ -45,6 +45,10 @@ Session tracking is only available when using Segment's new libraries: [Analytic
 Once you have a mapping, you can follow the steps in the Destinations Actions documentation on [Customizing mappings](/docs/connections/destinations/actions/#customizing-mappings).
 
 
+### Mappings for Device ID
+The Amplitude destination requires that each event includes either a Device ID or a User ID. If a User ID is not present, a Device ID will be used, and vice versa. By default, Segment maps the Segment property `context.device.id` to the Amplitude property `Device ID`. If `context.device.id` is not available, Segment will map the property `anonymousId` to the Amplitude `Device ID`. This is indicated by the following text in the Device ID field: `coalesce(` `context.device.id` `anonymousId` `)`. In future, you will be able to specify the default property and additional fallback properties should the higher-priority fallback(s) not be available.
+
+
 ### Enable session tracking for Analytics.js 2.0
 
 The session tracking is automatically enabled on Javascript sources.