ScyllaDB supports non-frozen UDTs since 3.2, no need to keep referencing
this limitation in the current docs. Replace the description of the
limitation with general description of frozen semantics for UDTs.
Fixes: #22929Closesscylladb/scylladb#24763
(cherry picked from commit 37ef9efb4e)
Closesscylladb/scylladb#24782