Note equivalency of NGINX config in Omnibus and Charts

What does this MR do?

Updates the charts/gitlab/webservice docs to mention which Omnibus NGINX's config item about HTTP body size is equivalent.

I'm going by https://github.com/gardener/gardener/issues/188#issuecomment-394956148 here, so the correctness of this MR needs to be checked by a K8S expert.

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