Make "WebTransport session" scoped (#308)

Before this change, #webtransport-session links to
"WebTransport session" and #webtransport-session① links to
"[[Session]]" for="WebTransport.

In order to have a public link to "[[Session]]" at
whatwg/html#6856, add "for=protocol" to
the link to "WebTransport session".

With this change #protocol-webtransport-session links to
"WebTransport session", and #webtransport-session links to
[[Session]].

This is for #127.

index.bs

@@ -142,8 +142,8 @@ The terms {{ReadableStream}} and {{WritableStream}} are defined in
 
 # Protocol concepts # {#protocol-concepts}
 
-A <dfn>WebTransport session</dfn> is a session of WebTransport over HTTP/3.
-There may be multiple [=WebTransport session=]s on one [=connection=], when pooling is enabled.
+A <dfn for="protocol">WebTransport session</dfn> is a session of WebTransport over HTTP/3.
+There may be multiple [=WebTransport sessions=] on one [=connection=], when pooling is enabled.
 
 [=WebTransport session=] has the following capabilities defined in [[!WEB-TRANSPORT-HTTP3]].