docs: clarify use of Extended CONNECT for h/2 (#13051)

Tweak the HTTP upgrades documentation to mention RFC8841 in the
documentation body (complementing the existing link to the RFC).

Minor fix to the warning text for CONNECT support.

Make explicit mention of "Extended CONNECT" in the API docs for
`RouteMatch`.

Closes #13044.

Signed-off-by: Nick Travers <n.e.travers@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 12d7b17906df69749990a9f481bc4fce087c22e6

envoy/config/route/v3/route_components.proto

@@ -440,7 +440,7 @@ message RouteMatch {
     // (WebSocket and the like) as they are normalized in Envoy as HTTP/1.1 style
     // upgrades.
     // This is the only way to match CONNECT requests for HTTP/1.1. For HTTP/2,
-    // where CONNECT requests may have a path, the path matchers will work if
+    // where Extended CONNECT requests may have a path, the path matchers will work if
     // there is a path present.
     // Note that CONNECT support is currently considered alpha in Envoy.
     // [#comment:TODO(htuch): Replace the above comment with an alpha tag.

envoy/config/route/v4alpha/route_components.proto

@@ -442,7 +442,7 @@ message RouteMatch {
     // (WebSocket and the like) as they are normalized in Envoy as HTTP/1.1 style
     // upgrades.
     // This is the only way to match CONNECT requests for HTTP/1.1. For HTTP/2,
-    // where CONNECT requests may have a path, the path matchers will work if
+    // where Extended CONNECT requests may have a path, the path matchers will work if
     // there is a path present.
     // Note that CONNECT support is currently considered alpha in Envoy.
     // [#comment:TODO(htuch): Replace the above comment with an alpha tag.