clarify nginx config setup

What does this MR do?

These changes are to clarify that the paths may need to be changed in the nginx config file depending on where gitlab is installed.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

There is an issue for it

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

This should address the confusion mentioned in #844 (closed)

I have used this merge request as the basis for these changes: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/382

Please let me know if any changes are required.

Also - this is my first merge request to gitlab-ce so any suggestions on how this could be better would be greatly appreciated!