I'm don't know what you mean by 'appropriate repo field'. I've emphasied there must be one or more Reviewers.
I could explicitly say that Differential is automatically adding "llvm-commits" to Subscribers.

Or do you mean something else?

I'm only highlighting "The mailing list should be added as a subscriber on all reviews" from https://llvm.org/docs/Phabricator.html
I think that a similar phrase really should be repeated here.

Ok, so instead of:

"It requires one or more Reviewers and allows zero or more Subscribers, finding-reviewer-names_."

How about:

"arc requires one or more Reviewers (finding-reviewer-names_) and Subscribers should be llvm-commits plus the appropriate -dev list(s) from listinfo."

I'll update llvm/docs/Phabricator-commit-message.png when I get it right :)

and Subscribers should be llvm-commits plus the appropriate -dev list(s) from listinfo."

Hm, not exactly.

"arc requires one or more Reviewers (finding-reviewer-names_) and Subscribers should be the appropriate -commits list(s) from listinfo."`

Although I normally add subscribers out of habit, Herald takes care of this for you when you submit a patch -- believe this is all maintained by @benhamilton.

There is a sufficient number of cases where that isn't so,
be it either intentionally going against the docs,
or just confusing the phabricator from auto-subscribing.

This leads to a different question.

I use git gui to amend the commits locally and then arc diff master to update. Do I need to manually update Subscriber with benhamilton so I don't accidentally remove it after @hintonda has added it?

Not sure I understand your question -- don't use gui's, but figure they can't be that different.

In any case, after you create a new patch in phab, you can update it by simply typeing arc diff. If you want to do it from somewhere other than where you originally ran the initial arc diff, you need to use the --update option, e.g., arc diff --update D61267.

You don't need to do anything else. For example, look at the first few lines of history at the top of this page. You'll see that Herald add the llvm project, and subscribed llvm-commits. It's a one-time thing you don't need to worry about.

As for @lebedev.ri comment about it not always working, personally, I'd consider that a bug and raise it on the list. I think @benhamilton fixed a few of those for me a couple of years ago. However, it doesn't hurt to mention it in the docs. hth... don

The repository field is set automatically based on the closest .arcconfig file going up from the directory in which you ran arc diff.

(I think there are scenarios where someone can run arc diff from a different directory or upload a diff directly to Phabricator which can bypass its detection of the repository, which could explain @lebedev.ri 's issues. Regardless, directly reporting problems can help.)

I'd add one more sentence here, to tell people that in the case that they are doing the currently-recommend thing, they do _NOT_ need to worry about this. Something like:
"If you are working off a clone of the official https://github.com/llvm/llvm-project/ repository, Phabricator will subscribe the correct lists automatically. Otherwise, please add the relevant ones yourself."

LGTM otherwise!