feat(server): allow embed with listen (#25709)

connection from any channel or stdio will unblock
remote_ui_wait_for_attach.  Wait on stdio only if
only —embed specified, if both —embed and
—listen then wait on any channel.

runtime/doc/starting.txt

@@ -390,6 +390,10 @@ argument.
 <		Then startup will continue without waiting for `nvim_ui_attach`.
 		This is equivalent to: >
 		    nvim --headless --cmd "call stdioopen({'rpc': v:true})"
+<
+		Embedders that use the UI protocol on a socket connection must
+		pass |--listen| as well as |--embed|: >
+		    nvim --embed --listen addr
 
 <		See also: |ui-startup| |channel-stdio|