Incorrect infromation in Upgrading Sync Gateway/Configuration Changes

On the page https://docs.couchbase.com/sync-gateway/current/upgrading.html#configuration-changes, we have incorrect usage of the 'import_docs' parameter. 

As discovered by the customer in the ticket https://supportal.couchbase.com/zendesk/ticket/50294

> The documentation says: After restarting Sync Gateway with the updated configuration file, the mobile metadata for existing documents is automatically migrated. To enable shared bucket on incoming documents, one Sync Gateway node in the cluster must also have the import_docs: continuous property in the configuration file.

However, in https://docs.couchbase.com/sync-gateway/current/configuration-schema-database.html#import_docs we describe 'import_docs' as a boolean variable, so 'continuous' value is impossible. 

Also, we need to mention the automatic migration of the metadata is only possible when 'import_docs' is set to true.