fix(151): clarify protobuf response for no parallel requests

[toumorokoshi]

For protobuf guidance, it was unclear response code should be.

Partially addresses #224

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[toumorokoshi]

@wora addressed comments! PTAL.

[dv-stewarts]

LGTM. I think splitting into tabs per protocol is the right answer. I'm not a reviewer, so cannot approve.

[rofrankel]

Suggested change

	{% tab proto %}
	- A resource that does not support concurrent mutation requests **must** return an
	`ABORTED` error if the resource has an outstanding mutation.
	{% tab oas %}
	- A resource that does not support concurrent mutation requests **must** respond
	with a `409 Conflict` status code if the resource has an outstanding mutation.
	{% endtabs %}
	- Resource that does not permit multiple requests in parallel (denying any new
	request until the one that is in progress finishes) **must** return
	`ABORTED`/`409 Conflict` if a user attempts a parallel request, and include an error
	message explaining the situation.

We discussed this live - seems unfortunate to add all the tab chrome to the page just to vary a single term. Thoughts?

[toumorokoshi]

I'm fine with consolidating it.

@wora gave feedback on the "include an error message" guidance earlier, and suggested that adding this type of guidance may result in users using the error string as a way to figure out the actual error (concurrent request). I removed that part intentionally based on that feedback (as I also agreed). Thoughts on that?