added callback notice to the migration guide

Author: Glynn Bird

migration_guide_v10_to_v11.md

@@ -1,13 +1,17 @@
 # Migrating from Nano 10 to Nano 11
 
-Nano 10 uses the Axios library as an HTTP/HTTPS client. Keeping up with changes to Axios and its dependencies made maintaining this library a chore, so as of Nano 11 we use Node.js's built-in _fetch_ API as our HTTP client. This makes Nano a _zero dependency_ library which makes for faster installs, easier maintenance and slightly better performance.
+The highlights:
 
-There are some things to bear in mind if you are switching from Nano 10 to Nano 11, so please consider the following advice carefully before you do.
+- Nano 11's HTTP client is replaced with the native Node.js _fetch_ function meaning that Nano 11 only works on Node.js 18 and above.
+- Nano no longer supports `requestDefaults` to configure the client options.
+- Nano no longer supports callbacks.
 
 ## Node.js versions
 
 > ** Nano 11 is a breaking change for users of Node.s 16 or earlier **
 
+Nano 10 uses the Axios library as an HTTP/HTTPS client. Keeping up with changes to Axios and its dependencies made maintaining this library a chore, so as of Nano 11 we use Node.js's built-in _fetch_ API as our HTTP client. This makes Nano a _zero dependency_ library which makes for faster installs, easier maintenance and slightly better performance.
+
 Nano 11 is only compatible with Node.js versions 18 and older, because it is only these Node versions that have the [fetch](https://nodejs.org/dist/latest-v18.x/docs/api/globals.html#fetch) HTTP client baked in. See [Node.js's Long-term Support page](https://nodejs.org/en/about/previous-releases) to see which are the currently supported and maintained versions. In short:
 
 - If you are using Node.js 18 or newer, use Nano 11.
@@ -17,6 +21,8 @@ Nano 10 may continue to receive some security fixes for a time, but Nano 11 repr
 
 ## Agent options
 
+> ** Nano 11 no longer supports `requestDefaults` to configure the HTTP client **
+
 None of Nano's API has changed _except_ when a user is supplying non-default connection handling parameters. Gone is `requestDefaults` which dates back to the "request" days and instead an optional `agentOptions` can be provided which is documented in the README and in TypeScript.
 
 ```js
@@ -43,3 +49,29 @@ const nano = Nano({ url: 'http://127.0.0.1:5984', undiciOptions })
 
 > Note: to supply a custom agent, you will need the [undici](https://www.npmjs.com/package/undici) module as a dependency in your own project. Undici is the library that powers Node.js's _fetch_ API but its "Agent" is not programmatically accessible unless undici is imported separately.
 
+## Callbacks
+
+> ** Nano 11 no longer supports callbacks **
+
+In Nano 10 and earlier, the last parameter to most Nano functions was a callback so you could do:
+
+```js
+db.list(function(err, data) { 
+  console.log('response', err, data)
+})
+```
+
+This was the way asynchronous activity was handled in the early days of Node.js but as of Nano 11 all callbacks are removed and we expect the use of Promises:
+
+```js
+db.list().then((data) => { console.log('response', data )})
+```
+
+or more commonly, the `await` pattern:
+
+```js
+const data = await db.list()
+console.log('response', data)
+```
+
+If your code makes use of the callback style then **it will not work with Nano 11**: you will need to rewrite your code to use Promises or await, or remain on Nano 10.