Document volume mount path for `datastoreTLSSecretName`

[ecordell]

It's mounted under spicedb-db-tls (which will need to be referenced in the datastore_uri in the secret), but this is not documented well.

[nico151999]

It took me a while to figure this out by myself. I had to do a deep dive until I found out that the operator does not automatically look for files named ca.crt, tls.crt and tls.key (i.e. keys with the same name in the secret specified for datastoreTLSSecretName). If I had seen this issue a bit earlier it would have saved me some time. Once I found this issue I just headed over to the cockroach documentation, added the necessary parameters pointing to the files in the /spicedb-db-tls directory and applied my changes. This worked like a charm then. So if there's anyone else finding this issue first before searching through the code I hope I can save them some time this way.