Fix redis details

As discussed at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/224 and add5d43b6ea0f4fbe85af4c07afe9e549b700205 the redis setup details aren't quite correct.

Currently the upgrade and installation guides differ in how you should set socket permissions. This PR aims to unify the settings both for a few old upgrade guides and, for users with already broken installations, in a new upgrade guide.

I will squash and make this mergeable if other users agree.

/cc @axilleas @hobarrera
Copied from 6.x-or-7.x-to-7.4.md & 7.2-to-7.3.md. What do you instead suggest it be. I can correct them all.