doc: clarify buffer toString docs.

Fixes: https://github.com/nodejs/node/issues/8971
PR-URL: https://github.com/nodejs/node/pull/8984
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.md

@@ -1811,12 +1811,13 @@ added: v0.1.90
 -->
 
 * `encoding` {String} The character encoding to decode to. **Default:** `'utf8'`
-* `start` {Integer} Where to start decoding. **Default:** `0`
-* `end` {Integer} Where to stop decoding (not inclusive). **Default:** [`buf.length`]
+* `start` {Integer} The byte offset to start decoding at. **Default:** `0`
+* `end` {Integer} The byte offset to stop decoding at (not inclusive).
+  **Default:** [`buf.length`]
 * Return: {String}
 
-Decodes `buf` to a string according to the specified character encoding in `encoding`.
-`start` and `end` may be passed to decode only a subset of `buf`.
+Decodes `buf` to a string according to the specified character encoding in
+`encoding`. `start` and `end` may be passed to decode only a subset of `buf`.
 
 Examples:
 
@@ -1829,19 +1830,22 @@ for (var i = 0 ; i < 26 ; i++) {
 }
 
 // Prints: abcdefghijklmnopqrstuvwxyz
-console.log(buf.toString('ascii'));
+console.log(buf1.toString('ascii'));
 
 // Prints: abcde
-console.log(buf.toString('ascii', 0, 5));
+console.log(buf1.toString('ascii', 0, 5));
 
 
 const buf2 = Buffer.from('tést');
 
-// Prints: tés
-console.log(buf.toString('utf8', 0, 3));
+// Prints: 74c3a97374
+console.log(buf2.toString('hex'));
+
+// Prints: té
+console.log(buf2.toString('utf8', 0, 3));
 
-// Prints: tés
-console.log(buf.toString(undefined, 0, 3));
+// Prints: té
+console.log(buf2.toString(undefined, 0, 3));
 ```
 
 ### buf.toJSON()