Explain how to move forward or backward from arc land --hold

Summary: Fixes T9973. When users run arc land --hold, give them the commands to move forward (push) or backward (checkout the branch/tag/commit they were on) and be explicit that branches have not changed.

Test Plan: Ran arc land --hold, got lots of explanatory text.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9973

Differential Revision: https://secure.phabricator.com/D14762