doc: fix, modernize examples in docs

* Use single quotes consistently
* Modernize examples to use template strings and arrow funcs
* Fix a few typos
* Example edits for consistency

PR-URL: https://github.com/nodejs/node/pull/4282
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/string_decoder.markdown

@@ -6,13 +6,13 @@ To use this module, do `require('string_decoder')`. StringDecoder decodes a
 buffer to a string. It is a simple interface to `buffer.toString()` but provides
 additional support for utf8.
 
-    var StringDecoder = require('string_decoder').StringDecoder;
-    var decoder = new StringDecoder('utf8');
+    const StringDecoder = require('string_decoder').StringDecoder;
+    const decoder = new StringDecoder('utf8');
 
-    var cent = new Buffer([0xC2, 0xA2]);
+    const cent = new Buffer([0xC2, 0xA2]);
     console.log(decoder.write(cent));
 
-    var euro = new Buffer([0xE2, 0x82, 0xAC]);
+    const euro = new Buffer([0xE2, 0x82, 0xAC]);
     console.log(decoder.write(euro));
 
 ## Class: StringDecoder