Doc: add a bit to indices.sgml about what is an indexable clause.

We didn't explain this clearly until somewhere deep in the
"Extending SQL" chapter, but really it ought to be mentioned
in the introductory material too.

Discussion: https://postgr.es/m/4097442.1694967650@sss.pgh.pa.us

doc/src/sgml/indices.sgml

@@ -90,6 +90,39 @@ CREATE INDEX test1_id_index ON test1 (id);
    also significantly speed up queries with joins.
   </para>
 
+  <para>
+   In general, <productname>PostgreSQL</productname> indexes can be used
+   to optimize queries that contain one or more <literal>WHERE</literal>
+   or <literal>JOIN</literal> clauses of the form
+
+<synopsis>
+<replaceable>indexed-column</replaceable> <replaceable>indexable-operator</replaceable> <replaceable>comparison-value</replaceable>
+</synopsis>
+
+   Here, the <replaceable>indexed-column</replaceable> is whatever
+   column or expression the index has been defined on.
+   The <replaceable>indexable-operator</replaceable> is an operator that
+   is a member of the index's <firstterm>operator class</firstterm> for
+   the indexed column.  (More details about that appear below.)
+   And the <replaceable>comparison-value</replaceable> can be any
+   expression that is not volatile and does not reference the index's
+   table.
+  </para>
+
+  <para>
+   In some cases the query planner can extract an indexable clause of
+   this form from another SQL construct.  A simple example is that if
+   the original clause was
+
+<synopsis>
+<replaceable>comparison-value</replaceable> <replaceable>operator</replaceable> <replaceable>indexed-column</replaceable>
+</synopsis>
+
+   then it can be flipped around into indexable form if the
+   original <replaceable>operator</replaceable> has a commutator
+   operator that is a member of the index's operator class.
+  </para>
+
   <para>
    Creating an index on a large table can take a long time.  By default,
    <productname>PostgreSQL</productname> allows reads (<command>SELECT</command> statements) to occur
@@ -120,7 +153,7 @@ CREATE INDEX test1_id_index ON test1 (id);
    B-tree, Hash, GiST, SP-GiST, GIN, BRIN, and the extension <link
    linkend="bloom">bloom</link>.
    Each index type uses a different
-   algorithm that is best suited to different types of queries.
+   algorithm that is best suited to different types of indexable clauses.
    By default, the <link linkend="sql-createindex"><command>CREATE
    INDEX</command></link> command creates
    B-tree indexes, which fit the most common situations.