doc: clarify where to find pg_type_d.h (PG 11+) and pg_type.h

These files are in compiled directories and install directories.

Reported-by: e.indrupskaya@postgrespro.ru

Discussion: https://postgr.es/m/160379609706.24746.7506163279454026608@wrigleys.postgresql.org

Backpatch-through: 9.5

Author: bmomjian

doc/src/sgml/libpq.sgml

@@ -3661,8 +3661,8 @@ 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>src/include/catalog/pg_type_d.h</filename>
-       in the source tree.
+       in the file <filename>include/catalog/pg_type_d.h</filename>
+       in the compiled source directory and install directory.
       </para>
      </listitem>
     </varlistentry>