docs: update the manual backup procedure to use `DESC SCHEMA WITH INTERNALS` instead of `DESC SCHEMA` about scylladb HOT 1 OPEN

It's also worth mentioning that this procedure requires proper support in cqlsh. The current implementation intercepts both DESC SCHEMA and DESC SCHEMA WITH INTERNALS and handles them in a special way, leading to wrong results for the latter query. This is already tracked in scylladb/scylla-cqlsh#17. Because using an old cqlsh version will lead to an incorrect schema dump which might not be sufficient to restore the schema later, it's very important to inform the users that they must use the right version of cqlsh (and tell them to check it before doing backup).

Is server-side describe used by Scylla and scylla-cqlsh at all?

Here in scylla-cqlsh: https://github.com/scylladb/scylla-cqlsh/blob/master/bin/cqlsh.py#L1593, cqlsh will do client-side describe when connection_versions['build'][0] < '4' and server-side describe otherwise.
But in Scylla: https://github.com/scylladb/scylladb/blob/master/version.hh#L27, version value is 3.0.8. connection_versions in cqlsh is not a the Scylla version (like upcoming 6.0).

@avelanarius I think we discussed a some time ago all requirements to bump up connection_versions and maintain compatibility with cassandra.
Cc @fruch

from scylladb.