doc: trim GitHub template comments

Make the comments in the GitHub templates slightly more concise.

PR-URL: https://github.com/nodejs/node/pull/6755


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>