site/upgrade.md: Address several comments from Gerhard
site/upgrade.md: Address several comments from Gerhard

Mainly, add more details and examples to several steps.

Signed-off-by: Daniil Fedotov <dfedotov@pivotal.io>

[#153172934]

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website