Add detail to backup_restore omnibus remote storage docs

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

What does this MR do?

Adds some important details to configuring remote storage with the backup_restore rake task, namely to add the configuration to /etc/gitlab/gitlab.rb and to make sure to run gitlab-ctl reconfigure after changing that file.

I burned a little bit of time with this, and at least one other person has run into this: #12929 (moved)

Moving docs to a new location?

Nope!