* lisp/org-agenda.el (org-agenda-archive-with): Bind
`org-archive-from-agenda' to `t' when archiving from agenda.
* lisp/org-archive.el (org-archive-subtree-save-file-p): New option.
(org-archive-subtree): Use the new option.
* doc/org-manual.org (Moving a tree to an archive file):
Mention the new option.
Thanks to Russ Allbery for suggesting a similar idea.