[ref guide] Fix description of field backupName for backup (#2554)

the previous description of backupName is not correct. Backup name should be the same if using incremental backups

solr/solr-ref-guide/modules/deployment-guide/pages/collection-management.adoc

@@ -1588,7 +1588,8 @@ s|Required |Default: none
 +
 What to name the backup that is created.
 Provided as a query parameter for v1 requests, or as a path segment for v2 requests.
-This is checked to make sure it doesn't already exist, and otherwise an error message is raised.
++
+For incremental backups, the backup name should be reused to add new backup points to the existing backup. For non-incremental backups (deprecated), this name is checked to ensure it doesn't already exist, and an error message is raised if it does. 
 
 `location`::
 +