Resolve conflict between BOLT 4&9 re: var_onion_optin context

BOLT 4 explicitly indicates var_onion_optin may appear in a BOLT 11
invoice, however, BOLT 9 only indicates it is available in init and
node_announcement contextx. Resolve this conflict in favor of BOLT 4
as there doesn't seem to be much reason to *not* allow it in BOLT
11 invoices.

09-features.md

@@ -32,7 +32,7 @@ The Context column decodes as follows:
 | 3     | `initial_routing_sync`           | Sending node needs a complete routing information dump    | I        | [BOLT #7][bolt07-sync]                |
 | 4/5   | `option_upfront_shutdown_script` | Commits to a shutdown scriptpubkey when opening channel   | IN       | [BOLT #2][bolt02-open]                |
 | 6/7   | `gossip_queries`                 | More sophisticated gossip control                         | IN       | [BOLT #7][bolt07-query]               |
-| 8/9   | `var_onion_optin`                | Requires/supports variable-length routing onion payloads  | IN       | [Routing Onion Specification][bolt04] |
+| 8/9   | `var_onion_optin`                | Requires/supports variable-length routing onion payloads  | IN9      | [Routing Onion Specification][bolt04] |
 | 10/11 | `gossip_queries_ex`              | Gossip queries can include additional information         | IN       | [BOLT #7][bolt07-query]               |
 | 12/13| `option_static_remotekey`         | Static key for remote output                              | IN       | [BOLT #3](03-transactions.md)         |
 | 14/15 | `payment_secret`                 | Node supports `payment_secret` field                      | IN9      | [Routing Onion Specification][bolt04]   |