doc: remove "above" and "below" references

The docs were recently refactored, and some "above" and "below"
references were no longer accurate. This commit removes many
such references, and replaces others with links.

PR-URL: https://github.com/nodejs/node/pull/4499


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>