Explain either IDs or names can be used for AWS resources

Adds a note to the manual explaining that either IDs or names can be
used when specifying instances or volumes in AWS.

doc/manual/28-snapshots.en.md

@@ -148,6 +148,10 @@ azure_resource_group = AZURE_RESOURCE_GROUP
 
 #### Configuration for AWS snapshots
 
+When specifying `snapshot_instance` or `snapshot_disks`, Barman will accept either the instance/volume ID which was assigned to the resource by AWS *or* a name.
+If a name is used then Barman will query AWS to find resources with a matching `Name` tag.
+If zero or multiple matching resources are found then Barman will exit with an error.
+
 The following optional parameters can be set when using AWS:
 
 ``` ini