update upgrade guide with backup, db specific commands, config changes, ...

Created by: bbodenmiller

...check, and restore

Closes #4047

Created by: bbodenmiller

replaces https://github.com/gitlabhq/gitlabhq/issues/4052, https://github.com/gitlabhq/gitlabhq/pull/4053, https://github.com/gitlabhq/gitlabhq/pull/4047, https://github.com/gitlabhq/gitlabhq/pull/4005, https://github.com/gitlabhq/gitlabhq/pull/4003

By Administrator on 2013-05-24T23:56:03 (imported from GitLab project)

By Administrator on 2013-05-24T23:56:03 (imported from GitLab)

Created by: senny

@randx can you take a look?

By Administrator on 2013-05-25T11:43:49 (imported from GitLab project)

By Administrator on 2013-05-25T11:43:49 (imported from GitLab)