doc: Add index entry for "multirange type"

Before now, looking up "multirange" in the index only led to the
multirange() function.  To make this more useful, also add an entry
pointing to the range types section.

doc/src/sgml/func.sgml

@@ -19206,7 +19206,7 @@ SELECT NULLIF(value, '(none)') ...
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
-         <primary>multirange</primary>
+         <primary>multirange (function)</primary>
         </indexterm>
         <function>multirange</function> ( <type>anyrange</type> )
         <returnvalue>anymultirange</returnvalue>

doc/src/sgml/rangetypes.sgml

@@ -7,6 +7,10 @@
   <primary>range type</primary>
  </indexterm>
 
+ <indexterm>
+  <primary>multirange type</primary>
+ </indexterm>
+
  <para>
   Range types are data types representing a range of values of some
   element type (called the range's <firstterm>subtype</firstterm>).