Improve the description of using arc land --revision

Summary: The message suggests that only one revision would land.

Test Plan: Read it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5277

src/workflow/ArcanistLandWorkflow.php

@@ -343,7 +343,8 @@ EOTEXT
         "'{$this->branch}' which are not present on '{$this->onto}':\n\n".
         $this->renderRevisionList($revisions)."\n".
         "Separate these revisions onto different {$this->branchType}s, or use ".
-        "'--revision <id>' to select one.";
+        "'--revision <id>' to use the commit message from <id> and land them ".
+        "all.";
       throw new ArcanistUsageException($message);
     }