doc: remove redundant warning information

process.emitWarning() already describes how to emit custom warnings,
so just merely provide a link to that function from the 'warning'
event documentation.

PR-URL: https://github.com/nodejs/node/pull/9590


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>