Inaccurate docstring test_boot_cloned_encrypted_volume

We need to update the docstring to accurately describe the two steps that are being performed. The first step is to destroy the instance, and the second step is to create an encrypted clone using the source_volid and then boot another instance from that cloned volume.

First in L125 the volume is not detached but the instance is destroyed. Second the test says that the test performs two clone operations. A non-encrypted clone and then an encrypted clone operation. Only the second is accurate, since the test only create a clone from the same volume type than the source volume which is encrypted. [1]

We need to update the docstring to something like this:

```
- Destroy instance
- Create encrypted clone using source_volid
- Boot another instance from cloned volume
```

[1] https://opendev.org/openstack/cinder-tempest-plugin/src/commit/0e9461155e2acf2d68f2b42c8a5942af8975190a/cinder_tempest_plugin/scenario/test_volume_encrypted.py#L126-L128