Fix instructions for upgrading Drupal core

[colans]

At http://docs.aegirproject.org/en/3.x/install/upgrade/#the-hostmaster-migrate-command, I added instructions for upgrading Drupal core to a new release if one doesn't want to wait for a new Aegir release (say to avoid security vulnerabilities).

However, it's a dangerous in-place recipe. Now that the changes from https://www.drupal.org/node/2698027 are in, we should update these instructions to use hostmaster-migrate instead. It'll now grab the latest stable Drupal core.

It needs more testing though as only @helmo has confirmed that this works.

[helmo]

We can for now add another command to run ... updb (although it's not required for 7.52)

1. Run database updates
    * `drush @hostmaster updatedb`

[helmo]

Added in f6ab423

[colans]

Shouldn't we switch these instructions to the safer hostmaster-migrate plan, as I mentioned in the description?

[helmo]

Feel free to ... but after almost 2 years I got the feeling that this would never get priority.

[colans]

Moved to https://gitlab.com/aegir/documentation/issues/3.