TODO: Remove a couple of since-completed items.

"notmuch tag" is implemented now and seems to work great (and fast).

As for the race condition, as noted in the description we're removing
it's not exposed directly in the API, but only in a client that
allows for looping over search results and removing the inbox tag
from all of them. But then, that's exactly what the "notmuch tag"
command does. So, as discussed, we've now documented that command
to highlight the issue. Problem resolved, (as well as we can).