cql3/statements/batch_statement.cc: improve batch size warning message
cql3/statements/batch_statement.cc: improve batch size warning message

Make the warning message clearer:
 * Include the number of partitions affected by the batch.
 * Be clear that the warning is about the batch size in bytes.

Fixes #7367

Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>

Closes #7417

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla