clarify how export/import names convert to JS strings (#569) (#573)

* When embedded in the web, clarify how export/import names convert to JS strings (#569)

* Fixes suggested by @jf

* Address more feedback

Added a link to http://monsur.hossa.in/2012/07/20/utf-8-in-javascript.html.  Simplified the decoding algorithm thanks to Luke's feedback.

Author: pizlonator

Web.md

@@ -28,6 +28,35 @@ WebAssembly's [modules](Modules.md) allow for natural [integration with
 the ES6 module system](Modules.md#integration-with-es6-modules) and allow
 synchronous calling to and from JavaScript.
 
+### Function Names
+
+A WebAssembly module imports and exports functions. WebAssembly names functions
+using arbitrary-length byte sequences. Any 8-bit values are permitted in a
+WebAssembly name, including the null byte and byte sequences that don't
+correspond to any Unicode code point regardless of encoding. The most natural
+Web representation of a mapping of function names to functions is a JS object
+in which each function is a property. Property names in JS are UTF-16 encoded
+strings. A WebAssembly module may fail validation on the Web if it imports or
+exports functions whose names do not transcode cleanly to UTF-16 according to
+the following conversion algorithm, assuming that the WebAssembly name is in a
+`Uint8Array` called `array`:
+
+```
+function convertToJSString(array)
+{
+  var string = "";
+  for (var i = 0; i < array.length; ++i)
+    string += String.fromCharCode(array[i]);
+  return decodeURIComponent(escape(string));
+}
+```
+
+This performs the UTF8 decoding (`decodeURIComponent(unescape(string))`) using
+a [common JS idiom](http://monsur.hossa.in/2012/07/20/utf-8-in-javascript.html).
+Transcoding failure is detected by `decodeURIComponent`, which may throw
+`URIError`. If it does, the WebAssembly module will not validate. This validation
+rule is only mandatory for Web embedding.
+
 ## Aliasing linear memory from JS
 
 If [allowed by the module](Modules.md#linear-memory-section), JavaScript can