Clarify behaviour of --enable-users and --overwrite-users flags for cbbackupmgr

The current descriptions for these flags are:

--enable-users

Enables restoring cluster level users. As backup/restore of users is only available for CB version 7.6 and upwards, the flag is ignored for previous versions. To work you will need the correct permissions (cluster.admin.security).

--overwrite-users

Overwrites the already existing users in the cluster as the default behavior of backup/restore of users is to skip already existing users, this flag can be used to overwrite this behavior. As backup/restore of users is only available for CB version 7.6 and upwards, the flag is ignored for previous versions.

We could clarify this further by stating explicitly what will happen in the case where a user is present in the cluster but not in the backup which is being restored (nothing, the user will not be touched).