Add atomic migrations to changelog and docs

CHANGELOG.md

@@ -17,6 +17,7 @@ IMPROVEMENTS:
  * api: Environment variables are ignored during service name validation [GH-3532]
  * cli: Allocation create and modify times are displayed in a human readable
    relative format like `6 h ago` [GH-3449]
+ * client: Sticky volume migrations are now atomic. [GH-3563]
  * client: Added metrics to track state transitions of allocations [GH-3061]
  * client: When `network_interface` is unspecified use interface attached to
    default route [GH-3546]

website/source/docs/job-specification/ephemeral_disk.html.md

@@ -41,7 +41,9 @@ job "docs" {
 - `migrate` `(bool: false)` - When `sticky` is true, this specifies that the
   Nomad client should make a best-effort attempt to migrate the data from a
   remote machine if placement cannot be made on the original node. During data
-  migration, the task will block starting until the data migration has completed.
+  migration, the task will block starting until the data migration has
+  completed. Migration is atomic and any partially migrated data will be
+  removed if an error is encountered.
 
 - `size` `(int: 300)` - Specifies the size of the ephemeral disk in MB.  The
   current Nomad ephemeral storage implementation does not enforce this limit;