This documentation should be present and apply to versions for which JCR Store has been removed (
To inform customers upgrading to such versions of LIferay from versions in which they used JCR Store that they must convert to a different store before attempting to upgrade.
JCR Store has been removed as of $Liferay_version_as_of_which_it_has_been_removed. If you intend to upgrade to this version of Liferay and in your current version you are using JCR Store, you must first convert from JCR Store to any of the other stores available before attempting to perform the upgrade.
You can convert from one store to another by going to Control Panel > Configuration > Server Administration > Data Migration.