Today there are a few places in the transport layer docs where we talk
about communication between nodes _within a cluster_. We also use the
transport layer for remote cluster connections, and these statements
also apply there, but this is not clear from today's docs. This commit
generalises these statements to make it clear that they apply to remote
cluster connections too.
It also adds a link from the docs on configuring TCP retries to the
(deeply-buried) docs on preserving long-lived connections.
cf0cab614d