doc: be more explicit in the sypnosis

Assuming less knowledge on the part of the reader, making it easier
to get start using Node.js.

PR-URL: https://github.com/nodejs/node/pull/17977
Fixes: https://github.com/nodejs/node/issues/17970

,
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>