Improve documentation on the system.large_* tables
Improve documentation on the system.large_* tables

This clarifies that "rows" are clustering rows and that there is no
information about individual collection elements.

The patch also documents some properties common to all these tables.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20190820171204.48739-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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