Clarify usage of Gitaly path when running on standalone Gitaly server

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.

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