doc: clarify how to find pg_type_d.h in the install tree

Followup to patch 152ed047.

Reported-by: Alvaro Herrera

Discussion: https://postgr.es/m/20201112202900.GA28098@alvherre.pgsql

Backpatch-through: 9.5

doc/src/sgml/libpq.sgml

@@ -3666,8 +3666,8 @@ Oid PQftype(const PGresult *res,
        You can query the system table <literal>pg_type</literal> to
        obtain the names and properties of the various data types. The
        <acronym>OID</acronym>s of the built-in data types are defined
-       in the file <filename>include/catalog/pg_type_d.h</filename>
-       in the compiled source directory and install directory.
+       in the file <filename>include/server/catalog/pg_type_d.h</filename>
+       in the install directory.
       </para>
      </listitem>
     </varlistentry>