doc: note that buf.buffer's contents might differ

This bit me in a personal project since I had no reason to read the
`buf.byteOffset` docs, so point readers there explicitly.

PR-URL: #29651
Reviewed-By: Anna Henningsen <[email protected]>

Author: strugee

doc/api/buffer.md

@@ -1025,6 +1025,9 @@ console.log(buf.toString('ascii'));
 * {ArrayBuffer} The underlying `ArrayBuffer` object based on
   which this `Buffer` object is created.
 
+This `ArrayBuffer` is not guaranteed to correspond exactly to the original
+`Buffer`. See the notes on `buf.byteOffset` for details.
+
 ```js
 const arrayBuffer = new ArrayBuffer(16);
 const buffer = Buffer.from(arrayBuffer);