doc: replace sane with reasonable

Removing ableist language because we don't need to make the comparison
to people with mental health issues to get our point across.

PR-URL: https://github.com/nodejs/node/pull/3980


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>