Correct docs re: rewriting indexes when table rewrite is skipped

Back in 367bc42 (for 9.2!) we "avoid[ed] index rebuild[ing] for
no-rewrite ALTER TABLE
.. ALTER TYPE." However the docs still claim that "a table rewrite is
not needed; but any indexes on the affected columns must still be
rebuilt."

I've attached a simple patch to update the docs to match the current behavior.

Thanks,
James Coleman