Arvados

From #21701#note-17:

The documentation says a valid source is:

<PDH>/<path> where <PDH> is the portable data hash of a collection on the cluster

I assume "on the cluster" means I can't refer to collections on other clusters in a federation. But I think you have to be pretty deep in Arvados, and read very carefully, to recognize that. If I followed right, should we specifically call this out under the "Usage restrictions" section?

Does this limitation also apply to PDHs in the manifest_text? If so, that seems worth calling out too.

Under the update method header, the description for replace_files reads:

Delete and replace files and directories using content from other collections

With these changes this seems to narrow. The argument can do more than "delete and replace files and directories," and those operations can work on multiple data sources, not just "other collections." It would be nice to broaden this. I also recognize that it's a challenge to do this without making it almost uselessly generic. "Update this collection's contents by referencing collection manifests" maybe?