[Docs] clarify that truncate is not transactional

[polarweasel]

From Kannan:

for DDLs and operations like truncate, we currently need them to be their own independent transactions. Postgres for example allows multiple DDLs also to be executed in same transaction but with YugabyteDB that's a restriction.

Oracle also for example, multi-DDL transactions aren't supported. Each DDL auto-commits.

The bottom line is that 'truncate' in our current implementation is not transactional.

We recommend against using:
a) truncate inside of a multi-step transaction
b) or running truncate concurrently with our read/write operations in the same table.

If this is mostly for CI/CD on smaller datasets, can you instead just use:
delete from T;
This is heavier weight (and also not recommended for very large data sets) but will be transactional.