Writeable CTEs documentation patch

Hi,

Attached is a documentation patch for writeable CTEs.

Most of it is explaining how this feature works in select.sgml.  I
wasn't sure if that's the right place, but couldn't find a better one.
I also wasn't able to find any place discussing the command tag, other
than libpq's documentation.  Is there one somewhere?

While working on the docs, I noticed one problem with the patch itself:
it doesn't handle multi-statement DO INSTEAD rules correctly.  I'm going
to submit a fix for that later.

Any suggestions, whatsoever, are welcome.


Regards,
Marko Tiikkaja