Update the scale-up document.

[ishan16696]

How to categorize this issue?
/area high-availability
/kind enhancement
/kind technical-debt

What would you like to be added:
As part of PR #777, we have removed the addition of the scale-up annotation gardener.cloud/scaled-to-multi-node for scaling up the etcd cluster on the etcd StatefulSet. so, it is necessary to update the scale-up documentation.

Corresponding issue on backup-restore:
gardener/etcd-backup-restore#920