Geo: Document how to make promote-to-primary-node work in multi-node setups

The current gitlab-ctl promote-to-primary-node command doesn't work in a multi-node HA setup, and it's not clear from the documentation what it's doing behind the scenes, which seems important in a recovery scenario.

First, we need to document that it does the following:

1. Promotes the PostgreSQL secondary to primary by touching the trigger file (/tmp/postgresql.trigger)
2. Removes the git user SSH keys (is this really necessary?)
3. Runs gitlab-ctl reconfigure (why?)
4. Runs gitlab-rake geo:set_secondary_as_primary

In my multi-node set up, I did the following:

1. Logged into the Geo secondary PostgreSQL DB and ran touch by hand
2. Logged into the Rails host and ran gitlab-rake geo:set_secondary_as_primary

I already had SSH keys configured the same for the primary and the secondary Geo installations, but I don't still fully understand why step 2 or 3 are needed.

/cc: @brodock