This commit removes the outdated information about seed nodes.
We no longer need it in the docs, as a) the documentation is versioned,
and b) the ScyllaDB Open Source 4.3 and ScyllaDB Enterprise 2021.1 versions
mentioned in the docs are no longer supported.
In addition, some clarification has been added to the existing sections.
Fixes https://github.com/scylladb/scylladb/issues/22400Closesscylladb/scylladb#23282
(cherry picked from commit dbbf9e19e4)
Closesscylladb/scylladb#23327
2ffbc81e19
