sstables: read: Document that primitive_consumer::read_32() is alloc-free
sstables: read: Document that primitive_consumer::read_32() is alloc-free

Callers will rely on it to assume that it does not invalidate
references to LSA objects.

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