Today we describe snapshots as "incremental" but their incrementality is
rather different beast from e.g. incremental filesystem backups. With
traditional backups you take a large and relatively infrequent "full"
backup and then a sequence of smaller "incremental" ones, and this whole
sequence of backups is required for a restore so it must be kept around
until at least the next full backup. In contrast, Elasticsearch
snapshots are logically independent and each can be deleted without
affecting the integrity of the others.
This distinction frequently causes confusion amongst newer users, so
this commit clarifies what we mean by "incremental" in the docs.
aa8ebeb918