doc: clarify how to read process.stdin

document more clearly that stdin will emit multiple readable events

PR-URL: https://github.com/nodejs/node/pull/27350


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: James M Snell <jasnell@gmail.com>