mergeback takes partial list

docs/README.commands

@@ -199,12 +199,16 @@ uploaded to the bikeshed and to packages imported from the base-suite.
 
 ** mergeback-bs
 Options: Name (R)
+         Package (O)
 
 If the base-suite of a bikeshed supports the "mergeback" option, this
-will try to "migrate" all packages from the bikeshed into the
-base-suite. The packages will have to follow all the usual constraints
-that apply to the base suite during normal package uploads (say,
-version constraints).
+will try to "migrate" packages from the bikeshed into the base-suite.
+The packages will have to follow all the usual constraints that apply
+to the base suite during normal package uploads (say, version
+constraints).
+
+If no Package line is given, all packages are merged back, otherwise
+only the packages listed.
 
 ** list-bs
 Options: None