doc: Mention requirement to --enable-tap-tests on section for TAP tests

Author: Greg Sabino Mullane
Discussion: https://postgr.es/m/CAKAnmmJYH2FBn_+Vwd2FD5SaKn8hjhAXOCHpZc6n4wXaUaW_SA@mail.gmail.com
Backpatch-through: 9.6

doc/src/sgml/regress.sgml

@@ -801,6 +801,8 @@ make check PROVE_TESTS='t/001_test1.pl t/003_test3.pl'
    <para>
     The TAP tests require the Perl module <literal>IPC::Run</literal>.
     This module is available from CPAN or an operating system package.
+    They also require <productname>PostgreSQL</productname> to be
+    configured with the option <option>--enable-tap-tests</option>.
    </para>
 
    <para>