CR-6989 OPENDJ-1946 Doc how to reinitialize replication

This patch adds a procedure to reinitialize data for a base DN.

This procedure uses the pre-/post-external-init subcommands,
rather than rm -rf of the changelogDb and database files.
The approach with rm is perhaps quicker for test/PoC environments,
but looks like it calls for shutting the service down.