doc: improve github templates by using comments

Use HTML comments to reduce potential noise in github templates.
Also, improve flow of the pull request, making it easier to read.

PR-URL: https://github.com/nodejs/node/pull/5710


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>