Improve :check_origin docs. (#6062)

Realized that passing only a host to `:check_origin` is not enough as the doc suggest,
since it might also check scheme, host, and port against a URI.

So this was not really working: `check_origin: ["my-app.fly.dev"]`
I had to include the scheme as well: `check_origin: ["https://my-app.fly.dev"]`.

The examples are clear but the rest of the doc is a bit misleading.

See https://github.com/phoenixframework/phoenix/blob/5dba0bb6216e26fe5c48aafebad64d5f96da8868/lib/phoenix/socket/transport.ex#L616-L618

lib/phoenix/endpoint.ex

@@ -828,7 +828,7 @@ defmodule Phoenix.Endpoint do
       if so, the level
 
     * `:check_origin` - if the transport should check the origin of requests when
-      the `origin` header is present. May be `true`, `false`, a list of hosts that
+      the `origin` header is present. May be `true`, `false`, a list of URIs that
       are allowed, or a function provided as MFA tuple. Defaults to `:check_origin`
       setting at endpoint configuration.
 
@@ -839,8 +839,8 @@ defmodule Phoenix.Endpoint do
       Only use in development, when the host is truly unknown or when
       serving clients that do not send the `origin` header, such as mobile apps.
 
-      You can also specify a list of explicitly allowed origins. Wildcards are
-      supported.
+      You can also specify a list of explicitly allowed origins. Each origin may include
+      scheme, host, and port. Wildcards are supported.
 
           check_origin: [
             "https://example.com",