Glossary: improve definition of "relation"

Define the more general term first, then the Postgres-specific meaning.

Wording from Tom Lane.

Discussion: https://postgr.es/m/CACJufxEZ48toGH0Em_6vdsT57Y3L8pLF=DZCQ_gCii6=C3MeXw@mail.gmail.com

doc/src/sgml/glossary.sgml

@@ -1419,11 +1419,15 @@
    <glossterm>Relation</glossterm>
    <glossdef>
     <para>
-     The generic term for all objects in a
-     <glossterm linkend="glossary-database">database</glossterm>
-     that have a name and a list of
-     <glossterm linkend="glossary-attribute">attributes</glossterm>
-     defined in a specific order.
+     Mathematically, a <firstterm>relation</firstterm> is a set of
+     <glossterm linkend="glossary-tuple">tuples</glossterm>;
+     this is the sense meant in the term "relational database".
+    </para>
+
+    <para>
+     In <productname>PostgreSQL</productname>, "relation" is commonly used to
+     mean an <glossterm linkend="glossary-sql-object">SQL object</glossterm>
+     that has a name and a list of attributes defined in a specific order.
      <glossterm linkend="glossary-table">Tables</glossterm>,
      <glossterm linkend="glossary-sequence">sequences</glossterm>,
      <glossterm linkend="glossary-view">views</glossterm>,
@@ -1431,15 +1435,14 @@
      <glossterm linkend="glossary-materialized-view">materialized views</glossterm>,
      composite types, and
      <glossterm linkend="glossary-index">indexes</glossterm> are all relations.
+     A relation in this sense is a container or a descriptor for a set of tuples.
     </para>
+
     <para>
-     More generically, a relation is a set of tuples; for example,
-     the result of a query is also a relation.
-    </para>
-    <para>
-     In <productname>PostgreSQL</productname>,
-     <firstterm>Class</firstterm> is an archaic synonym for
-     <firstterm>relation</firstterm>.
+     <firstterm>Class</firstterm> is an alternative but archaic term.
+     The system catalog
+     <link linkend="catalog-pg-class"><structname>pg_class</structname></link>
+     holds an entry for each <productname>PostgreSQL</productname> relation.
     </para>
    </glossdef>
   </glossentry>