Document how a user should perform an offline non-root upgrade.

Background
The vbucket database file format is effectively changing in mad-hatter, the mad-hatter KV-engine will refuse to warm-up if the datafiles are found to be in the "old" format.

The file-format change though is handled automatically

• online upgrade, as data moves about via rebalance it will be written in the new format.
• offline upgrade, the installer runs "cbupgrade" before starting couchbase. This is a script which will do the conversion

Issue

We document how to perform a non-root install, but not the upgrade.

When we do the non-root install, the cbupgrade script is not executed, which is fine for a brand new install, however the cbupgrade script will need running if upgrading to mad-hatter.

I tried myself to do an offline-non-root upgrade but could not make it work without losing the node's config.

I don't know if there is a correct set of steps which needs documenting or it could be that offline non-root upgrade isn't possible/supported (which should be made clear in the non-root install)