I don't know if the certificate, itself, would help much, maybe a little, if it gets you past some resume filters. Experience and examples of past work (i.e. a portfolio) will trump just a certificate.
If you're interested in open source projects, look at some with great documentation and find some that have awful or non-existent documentation. Then see if you can find one of the bad ones that you could make look like one of the good ones. That would provide great experience and a wonderful portfolio piece (you'd have an example of analysis, authoring, design, customer/community feedback, etc.)
And, it wouldn't cost a cent (a lot of time, yes, but no money).
Related advice: find a mentor and/or an editor who can help you on your journey.