doc: note http.closeAllConnections excludes upgraded sockets

PR-URL: #53560
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>

doc/api/http.md

@@ -1705,8 +1705,10 @@ setTimeout(() => {
 added: v18.2.0
 -->
 
-Closes all connections connected to this server, including active connections
-connected to this server which are sending a request or waiting for a response.
+Closes all established HTTP(S) connections connected to this server, including
+active connections connected to this server which are sending a request or
+waiting for a response. This does _not_ destroy sockets upgraded to a different
+protocol, such as WebSocket or HTTP/2.
 
 > This is a forceful way of closing all connections and should be used with
 > caution. Whenever using this in conjunction with `server.close`, calling this