doc: clarify out-of-bounds behavior of buf[index]

PR-URL: https://github.com/nodejs/node/pull/11286
Fixes: https://github.com/nodejs/node/issues/11244
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>

doc/api/buffer.md

@@ -947,6 +947,10 @@ The index operator `[index]` can be used to get and set the octet at position
 `index` in `buf`. The values refer to individual bytes, so the legal value
 range is between `0x00` and `0xFF` (hex) or `0` and `255` (decimal).
 
+This operator is inherited from `Uint8Array`, so its behavior on out-of-bounds
+access is the same as `UInt8Array` - that is, getting returns `undefined` and
+setting does nothing.
+
 Example: Copy an ASCII string into a `Buffer`, one byte at a time
 
 ```js