Retire a shared node in use with ?detach=true

DELETE /flows/library/{name} still answers 409 while nodes use the source.
With detach=true each of them keeps a copy of the code, in the published
flow and the draft, the source is deleted and the running users rebuild.

docs/code/api.md

@@ -83,6 +83,9 @@ Ordinary source edits stay in the draft until publishing. Shared library
 source changes affect its users immediately.
 `POST /flows/{name}/nodes/{id}/share` promotes a source to the library;
 `/unshare` makes it private again. `GET /flows/library` lists shared sources.
+`DELETE /flows/library/{name}` retires one, and answers 409 while nodes still
+use it. With `?detach=true` each of those nodes keeps a copy of the code, in
+the published flow and the draft, and the shared source is deleted.
 
 ### Running and controlling