tools: fix comment nits in tools/doc/*.js files

* Unify first letters case.
* Unify periods.
* Delete excess spaces.
* Add some blank lines as logical delimiters.
* Remove obvious comments.
* Combine short lines, rewrap lines more logically.
* Fix typos.
* "XXX" -> "TODO:", OSX -> macOS.

PR-URL: https://github.com/nodejs/node/pull/19696


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>