This commit adjusts a `deprecation[...]` message in the docs since such
messages must be on a single line. It also moves this message to the start of
the description of the deprecated setting as is the case with other such
messages.
This commit updates the docs about translog retention and flushing to reflect
recent changes in how peer recoveries work. It also adds some docs to describe
how history is retained for replay using soft deletes and shard history
retention leases.
Relates #45473
David Turner
Nhat Nguyen