thrift: Add explanatory note about describe_splits
thrift: Add explanatory note about describe_splits

We don't implement describe_splits, and this patch describes why that
it. In a nutshell, to properly implement this, we would need something
like Origin's SizeEstimatesRecorder.java, but as the verb is marked as
experimental, we don't do it for now.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

duarten/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla