Improve upgrade documentation to include content about autopilot configuration settings

When using Consul enterprise, the autopilot setting `autopilot_disable_upgrade_migration` defaults to false. When this setting is false, Consul expects operators to use a blue/green upgrade strategy where if there are n servers, n additional servers are added with the new version, and the autopilot feature will downgrade previous version servers to non-voters while upgrading new version servers to voters.

However, if the operator adds one server at a time to do a manual rolling upgrade, this setting causes leader election to stop due to loss of quorum, preventing the upgrade from proceeding.

We should improve our [upgrade docs](https://www.consul.io/docs/upgrading#upgrading-consul) to call out enterprise specific features and how different upgrade strategies are impacted by autopilot settings.