Documentation improvements

* Use actual function name in section id
* More descriptive title
* Explain what expression the function computes and that's a boolean
* GIST -> GiST

Author: df7cb

doc/functions.sgm

@@ -149,9 +149,9 @@
             </example>
           </sect2>
 
-          <sect2 id="func.spoint.dist">
+          <sect2 id="func.spoint_dwithin">
             <title>
-              Point distance function
+              Point-within-distance function
             </title>
             <para>
               The function
@@ -166,8 +166,9 @@
             </funcsynopsis>
             <para>
               returns if points <parameter>p1</parameter> and <parameter>p2</parameter>
-              lie within distance <parameter>radius</parameter> of each other.
-              On PostgreSQL 12 and later, the function has <literal>GIST</literal>
+              lie within distance <parameter>radius</parameter> of each other, i.e.
+              it computes the boolean expression <literal>p1 &lt;-> p2 &lt;= radius</literal>.
+              On PostgreSQL 12 and later, the function has <literal>GiST</literal>
               support and the PostgreSQL optimizer will transform it to either
               <literal>p1 &lt;@ scircle(p2, radius)</literal> or
               <literal>p2 &lt;@ scircle(p1, radius)</literal> where appropriate.