Fix docs for invoice_payment hook and notification

The data-format changed in 2019.
The field `amount_msat` was renamed to `msat`
(See 2907e43)

The data-format changed again in november 2023.
The content of the field `amount_msat` was changed.
It was a string `"1000msat"` and became a number `1000`.
(See 0b23133)

The current documentation still mentions the data-format before
2907e43.

I've updated the docs to reflect the latest changes.
I've also incluced a short mention of 0b23133 to help help
developers create plugins that work both pre 23.11 and post 23.11.

doc/developers-guide/plugin-development/event-notifications.md

@@ -114,12 +114,14 @@ A notification for topic `invoice_payment` is sent every time an invoice is paid
   "invoice_payment": {
     "label": "unique-label-for-invoice",
     "preimage": "0000000000000000000000000000000000000000000000000000000000000000",
-    "amount_msat": 10000
+    "msat": 10000,
   }
 }
 
 ```
 
+Before version `23.11` the `msat` field was a string with msat-suffix, e.g: `"10000msat"`.
+
 Note that there will be a string member "outpoint" ("txid:outnum") if
 the payment was onchain (possible with the *invoices-onchain-fallback*
 config option).

doc/developers-guide/plugin-development/hooks.md

@@ -163,12 +163,11 @@ This hook is called whenever a valid payment for an unpaid invoice has arrived.
   "payment": {
     "label": "unique-label-for-invoice",
     "preimage": "0000000000000000000000000000000000000000000000000000000000000000",
-    "amount_msat": 10000
+    "msat": 10000
   }
 }
 ```
-
-
+Before version `23.11` the `msat` field was a string with msat-suffix, e.g: `"10000msat"`.
 
 The hook is deliberately sparse, since the plugin can use the JSON-RPC `listinvoices` command to get additional details about this invoice. It can return a `failure_message` field as defined for final nodes in [BOLT 4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md#failure-messages), a `result` field with the string
 `reject` to fail it with `incorrect_or_unknown_payment_details`, or a `result` field with the string `continue` to accept the payment.