Update p2 specification regarding repository url

The URL should only be an optional attribute, given that a p2 artifact may "travel along" and therefore the host repository is
not always clear/known.

PURL-TYPES.rst

@@ -402,8 +402,10 @@ p2
 ``p2`` for Eclipse p2 packages:
 
 - As each project is allowed to host their own update site, there is no default
-  repository. The base path to the repository has to be provided via the 
-  ``repository_url`` qualifier key. Examples::
+  repository. The base path to the repository is provided by the
+  ``repository_url`` qualifier key. This key is optional, given that the 
+  repository an artifact is located in might differ from the one it was
+  originally hosted in (e.g. SimRel/Orbit repositories). Examples::
 
        https://download.eclipse.org/releases/2023-09/202309131000
        https://download.itemis.com/updates/releases/2.1.1