$gzip_ratio ratio needs documentation

[Jens Rantil]

I propose two improvements to this wiki page: ​http://wiki.nginx.org/HttpGzipModule

1. Create a "Variables" section just like ​http://wiki.nginx.org/HttpCoreModule has and add the $gzip_ratio variable to it. Having the same structure on multiple wiki pages will improve browsability/readability. I missed the variable a couple of times because I simply was not lookin in the intro paragraph.
2. Document exactly how the ratio is calculated. I guess it is origsize/compressedsize?

[Ruslan Ermilov]

1. FYI, official English documentation is here: ​http://nginx.org/en/docs/http/ngx_http_gzip_module.html, but it also misses the "Embedded Variables" section, like many other modules. Wiki page mainly resembles its contents.

2. The data compression ratio is computed ​traditionally => need not to be documented explicitly.

[Valentin V. Bartenev]

1. Since it's a Wiki page, please feel free to improve it.

[vl]

Documentation updated - gzip_ratio description added: ​http://hg.nginx.org/nginx.org/rev/ae15915990ad.