doc: document that sendonion doesn't have to specify (but can!) the f…

…irst_hop channel.

This was always the case, though it was ignored.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-sendonion.7.md

@@ -42,6 +42,10 @@ to add an HTLC for 1002 millisatoshis and a delay of 21 blocks on top of the cur
 }
 ```
 
+If the first element of *route* does not have "channel" set, a
+suitable channel (if any) will be chosen, otherwise that specific
+short-channel-id is used.
+
 The *payment_hash* parameter specifies the 32 byte hex-encoded hash to use as
 a challenge to the HTLC that we are sending. It is specific to the onion and
 has to match the one the onion was created with.