doc: howto decode buffers extending from Writable

Improved stream documentation with an example of how to decode buffers
to strings within a custom Writable.

Fixes: nodejs/node#15369
PR-URL: nodejs/node#16403
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/api/stream.md

@@ -1510,6 +1510,47 @@ class MyWritable extends Writable {
 }
 ```
 
+#### Decoding buffers in a Writable Stream
+
+Decoding buffers is a common task, for instance, when using transformers whose
+input is a string. This is not a trivial process when using multi-byte
+characters encoding, such as UTF-8. The following example shows how to decode
+multi-byte strings using `StringDecoder` and [Writable][].
+
+```js
+const { Writable } = require('stream');
+const { StringDecoder } = require('string_decoder');
+
+class StringWritable extends Writable {
+  constructor(options) {
+    super(options);
+    const state = this._writableState;
+    this._decoder = new StringDecoder(state.defaultEncoding);
+    this.data = '';
+  }
+  _write(chunk, encoding, callback) {
+    if (encoding === 'buffer') {
+      chunk = this._decoder.write(chunk);
+    }
+    this.data += chunk;
+    callback();
+  }
+  _final(callback) {
+    this.data += this._decoder.end();
+    callback();
+  }
+}
+
+const euro = [[0xE2, 0x82], [0xAC]].map(Buffer.from);
+const w = new StringWritable();
+
+w.write('currency: ');
+w.write(euro[0]);
+w.end(euro[1]);
+
+console.log(w.data); // currency: €
+```
+
 ### Implementing a Readable Stream
 
 The `stream.Readable` class is extended to implement a [Readable][] stream.