doc: general improvements to string_decoder.md copy

doc/api/string_decoder.md

@@ -2,9 +2,15 @@
 
     Stability: 2 - Stable
 
-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.
+The `string_decoder` module provides an API for decoding `Buffer` objects into
+strings in a manner that preserves encoded multi-byte UTF-8 and UTF-16
+characters. It can be accessed using:
+
+```js
+const StringDecoder = require('string_decoder').StringDecoder;
+```
+
+The following example shows the basic use of the `StringDecoder` class.
 
 ```js
 const StringDecoder = require('string_decoder').StringDecoder;
@@ -17,23 +23,55 @@ const euro = Buffer.from([0xE2, 0x82, 0xAC]);
 console.log(decoder.write(euro));
 ```
 
-## Class: StringDecoder
+When a `Buffer` instance is written to the `StringDecoder` instance, an
+internal buffer is used to ensure that the decoded string does not contain
+any incomplete multibyte characters. These are held in the buffer until the
+next call to `stringDecoder.write()` or until `stringDecoder.end()` is called.
+
+In the following example, the three UTF-8 encoded bytes of the European euro
+symbol are written over three separate operations:
+
+```js
+const StringDecoder = require('string_decoder').StringDecoder;
+const decoder = new StringDecoder('utf8');
+
+decoder.write(Buffer.from([0xE2]));
+decoder.write(Buffer.from([0x82]));
+console.log(decoder.end(Buffer.from([0xAC])));
+```
+
+## Class: new StringDecoder([encoding])
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-Accepts a single argument, `encoding` which defaults to `'utf8'`.
+* `encoding` {string} The character encoding the `StringDecoder` will use.
+  Defaults to `'utf8'`.
 
-### decoder.end()
+Creates a new `StringDecoder` instance.
+
+### stringDecoder.end([buffer])
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-Returns any trailing bytes that were left in the buffer.
+* `buffer` {Buffer} A `Buffer` containing the bytes to decode.
+
+Returns any remaining input stored in the internal buffer as a string. Bytes
+representing incomplete UTF-8 and UTF-16 characters will be replaced with
+substitution characters appropriate for the character encoding.
 
-### decoder.write(buffer)
+If the `buffer` argument is provided, one final call to `stringDecoder.write()`
+is performed before returning the remaining input.
+
+### stringDecoder.write(buffer)
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-Returns a decoded string.
+* `buffer` {Buffer} A `Buffer` containing the bytes to decode.
+
+Returns a decoded string, ensuring that any incomplete multibyte characters at
+the end of the `Buffer` are omitted from the returned string and stored in an
+internal buffer for the next call to `stringDecoder.write()` or
+`stringDecoder.end()`.