diff/dump/restore: describe snapshot:subfolder syntax in help text

cmd/restic/cmd_diff.go

@@ -29,6 +29,10 @@ directory:
 * M  The file's content was modified
 * T  The type was changed, e.g. a file was made a symlink
 
+To only compare files in specific subfolders, you can use the
+"<snapshotID>:<subfolder>" syntax, where "subfolder" is a path within the
+snapshot.
+
 EXIT STATUS
 ===========
 

cmd/restic/cmd_dump.go

@@ -27,6 +27,10 @@ Pass "/" as file name to dump the whole snapshot as an archive file.
 The special snapshot "latest" can be used to use the latest snapshot in the
 repository.
 
+To include the folder content at the root of the archive, you can use the
+"<snapshotID>:<subfolder>" syntax, where "subfolder" is a path within the
+snapshot.
+
 EXIT STATUS
 ===========
 

cmd/restic/cmd_restore.go

@@ -28,6 +28,9 @@ a directory.
 The special snapshot "latest" can be used to restore the latest snapshot in the
 repository.
 
+To only restore a specific subfolder, you can use the "<snapshotID>:<subfolder>"
+syntax, where "subfolder" is a path within the snapshot.
+
 EXIT STATUS
 ===========