Update index.md to ellaborate on port configuration and accessing Prometheus from non-localhost hosts.

username-removed-53094 requested to merge nathan.jones/gitlab-ce:patch-24 into master

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Previously, the documentation did not (to my knowledge) mention that Prometheus was designed "to only listen on localhost by default", and has since been updated to allow for non-localhost hosts to access it. The documentation now accounts for this and describes the configuration required to allow for external access to the Prometheus service.

Edited by username-removed-53094

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