Table Function (aka SRF) doc patch

Here's the first doc patch for SRFs. The patch covers general
information and SQL language specific info wrt SRFs. I've taken to
calling this feature "Table Fuctions" to be consistent with (at least)
one well known RDBMS.

Note that I mention under the SQL language Table Function section that
"Functions returning sets" in query target lists is a deprecated
feature, subject to removal in later releases. I think there was general
agreement on this, but I thought it was worth pointing out.

I still need to submit some C language function documentation, but was
hoping to see if any further changes were needed in the Composite and
SRF API patch that I sent in earlier. I've started the documentation but
will hold of sending in a patch for now on that.

If no objections, please apply.

Thanks,

Joe

p.s. any feedback on the SRF regression test patch?