Unfortunately, this is awkward: we just copy through most requests,
so we can't easily add a "deprecation" field to each one. So we do
a notification if the next command has a different deprecation status
than the global one, but that requires opt-in from the plugin.
We didn't previously document the subscriptions array, so do that now.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: Plugins: `deprecated_oneshot` notifiction subscription to change deprecated status for a single command.
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