doc: clarify timing of socket.connecting

Fixes: https://github.com/nodejs/node/issues/25328

PR-URL: https://github.com/nodejs/node/pull/25333


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>