doc: make comment indentation consistent

Currently, some of the docs use different indentation for comments
in the code examples. This commit makes the indentation consistent
by putting the comments at the beginning of the line (really no
indentation that is).

PR-URL: https://github.com/nodejs/node/pull/9518


Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <targos@protonmail.com>