doc: fix nits in tools/doc/README.md

* Sync format schemes with the current doc state.
* Lowercase primitive types.
* Fix typos and unify the style.
* Remove tautological info.

PR-URL: https://github.com/nodejs/node/pull/18874


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>