Relax snapshot repo backup docs

It's now ok to mark a repository as `readonly: true` while taking a
backup, so this commit reinstates the docs that say this.

Relates elastic#93575
Reverts elastic#115062
Reverts elastic#115071
Relates elastic#128296

Author: DaveCTurner

docs/reference/snapshot-restore/register-repository.asciidoc

@@ -251,8 +251,8 @@ You must ensure that {es} does not write to the repository while you are taking
 the backup of its contents. If {es} writes any data to the repository during
 the backup then the contents of the backup may not be consistent and it may not
 be possible to recover any data from it in future. Prevent writes to the
-repository by unregistering the repository from the cluster which has write
-access to it.
+repository by unregistering it, or by registering it with `readonly: true`, on
+all your clusters.
 
 Alternatively, if your repository supports it, you may take an atomic snapshot
 of the underlying filesystem and then take a backup of this filesystem