Updated docs in 5-1-stable. Fixes #3757 and #3639.

- New mysql privilege:
There has been added a new permission in 5.1 that isn't included on update.md. Without that, backup fails. See [this](https://groups.google.com/forum/?fromgroups=#!topic/gitlabhq/So4mYIRFsX8) forum post.

- Fixed path for `init.d/gitlab` in rake check. Fixes #3757.

- On a new installation create `tmp/sockets` directory. Fixes #3639.