Documentation fix for ALTER TYPE .. RENAME

The documentation for ALTER TYPE .. RENAME claimed to support a
RESTRICT/CASCADE option at the 'type' level, which wasn't implemented
and doesn't make a whole lot of sense to begin with.  What is supported,
and previously undocumented, is

ALTER TYPE .. RENAME ATTRIBUTE .. RESTRICT/CASCADE.

I've updated the documentation and back-patched this to 9.1 where it was
first introduced.