1945102ca0
Fix some problems in the documentation, e.g. it is not possible to
enable Raft in an existing cluster in 5.0, but the documentation claimed
that it is.
(cherry picked from commit 1cc68b262e)
Cherry-pick note: the original commit added a lot of new stuff like
describing the Raft upgrade procedure, but also fixed problems with the
existing documentation. In this backport we include only the latter.
Closes #12582
