doc: set module version 72 to node 12

Clearly state the modules version 72 is official node 12, similar as it
is done for other major node versions.

PR-URL: https://github.com/nodejs/node/pull/29877


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>