[css-navigation-1] Drop non-useful descriptors and note possible future changes. (#13249)

Author: dbaron

css-navigation-1/Overview.bs

@@ -57,8 +57,8 @@ based on the following definitions:
 <dfn><<pattern-descriptor>></dfn> = pattern : <<url-pattern()>>
 <dfn><<init-descriptors>></dfn> = ;* <<init-descriptor>> [ ;+ <<init-descriptor>> ]* ;*
 <dfn><<init-descriptor>></dfn> = <<init-descriptor-name>> : <<string>>
-<dfn><<init-descriptor-name>></dfn> = protocol | username | password | hostname | port
-                         pathname | search | hash | base-url
+<dfn><<init-descriptor-name>></dfn> = protocol | hostname | port | pathname |
+                         search | hash | base-url
 </pre>
 
 This associates an author-defined keyword with a URL pattern,
@@ -89,6 +89,21 @@ for the route names?
 ISSUE: Is there value in being able to provide a list of <<url-pattern()>>
 rather than just one?
 
+NOTE: The list of allowed init descriptors does not include <code>username</code>
+or <code>password</code> since they seem unlikely to be useful.
+
+<div class="issue">
+It's possible that this syntax with init descriptors in the ''@route'' rule
+would make more sense as part of the ''urlpattern()'' function
+(that is, as an alternate syntax for what goes inside the function).
+
+This would also give us the option to remove the braces from
+the syntax of the ''@route'' rule
+and make it more like ''@import'' or ''@namespace''.
+This does remove a potential future extensibility point,
+but it could also be added back later if we need it.
+</div>
+
 <div class="example">
 Either this rule:
 <pre highlight=css>