doc: document use of buffer.swap16() for utf16be

Fixes: #12813

PR-URL: #23747
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: jasnell

doc/api/buffer.md

@@ -1933,6 +1933,14 @@ buf2.swap16();
 // Throws ERR_INVALID_BUFFER_SIZE
 ```
 
+One convenient use of `buf.swap16()` is to perform a fast in-place conversion
+between UTF-16 little-endian and UTF-16 big-endian:
+
+```js
+const buf = Buffer.from('This is little-endian UTF-16', 'utf16le');
+buf.swap16(); // Convert to big-endian UTF-16 text.
+```
+
 ### buf.swap32()
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