Doc: update ddl.sgml's description of cmin and cmax.

Doc: update ddl.sgml's description of cmin and cmax.

We long ago folded these two tuple header fields into one field
to save space.  However, nothing was done to the user-facing
documentation about them, perhaps with the idea that we'd add
code to emit something approximating the original definitions.
That never happened and presumably never will, so update the
text to reflect current reality.

Author: Paul A Jungwirth <pj@illuminatedcomputing.com>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA+renyVYYboiTayRRE0j1oKpeB+NjEBSUXfwgEu6O0JESSmauQ@mail.gmail.com