[Playlist] new language on URIs, URNs, URLs

[Lucas Gonze]

New language in terminology section:
...
2.3.1 URI, URLs and URNs

We use the term URI in this document regardless of whether a given URI 
specifies the location of a resource (a URL) or its name (a URN) 
independent of location.

See also: RFC2396bis; URIs, URLs, and URNs: Clarifications and 
Recommendations 1.0
...

New language in identifier definition:
...
4.1.1.2.14.1.1.1.2 identifier

Canonical ID for this resource. Likely to be a hash or other 
location-independent name, such as a MusicBrainz identifier. MUST be a 
legal URI. xspf:playlist elements MAY contain zero or more identifier 
elements.

For example, the URI 
http://musicbrainz.org/track/7e1d6f5f-0ac3-4889-8b57-506a67b459fc.html 
is an identifier for a specific song, but dereferencing that identifier 
will not yield a copy of the song.
...

Also:
I replaced most instances of the terms "URN" and "URL" with the term "URI."