sql: fix the batching behavior of mutation statements, especially with RETURNING

[jseldess]

PR: cockroachdb/cockroach#23373

From release notes:

The EXPLAIN output for UPDATE statements now also report CHECK expressions, for consistency with INSERT. [#23373][#23373] {% comment %}doc{% endcomment %}

[rmloveland]

@knz It's my opinion that this issue does not require a doc update. Do you agree with that assessment?

For context, I played with the EXPLAIN DELETE FROM ... and EXPLAIN INSERT INTO ... output from the tests a bit, but AFAICT we don't currently document the EXPLAIN output at the "statement X is EXPLAINed as output Y" level of granularity. Please let me know if I'm misunderstanding something here, though.

[knz]

It's my opinion that this issue does not require a doc update. Do you agree with that assessment?

correct!