Clarify usage of Gitaly path when running on standalone Gitaly server
### Problem to solve
https://gitlab.com/gitlab-org/gitlab-ce/commit/0923fbf75be44770b8ce01321cd304662e15d5d5 removes example of using `path` in Gitaly config and states `` In some cases, you'll have to set `path` for `gitaly['storage']` `` yet gives no details on what those cases are.
### Further details
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### Proposal
* [x] address inconsistent wording: "you need to configure storage paths"... then shows no example of setting storage `path`
* [ ] address irrelevant note: "In most or all cases, the storage paths below end in `/repositories` which is not that case"... again no example of setting `path` is even shown
* [ ] state what happens by default when `path` is excluded
* [ ] state when `path` should be included or excluded
* [ ] update https://gitlab.com/charts/gitlab/blob/master/doc/advanced/external-gitaly/external-omnibus-gitaly.md#configure-omnibus-gitlab as is supposed to happen when https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/administration/gitaly/index.md is updated
### Who can address the issue
@eread @axil @zj-gitlab
### Other links/references
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issue