doc: Mention modules/injection_points as example for injection points

This should have been added in 49cd2b93d7db, that introduced the module.

Reported-by: Jian He
Discussion: https://postgr.es/m/CACJufxF+Vfj2Oz2kBR5v1bjHeZxvs63cLogm70v9Uto1Rqiieg@mail.gmail.com

doc/src/sgml/xfunc.sgml

@@ -3668,6 +3668,12 @@ extern bool InjectionPointDetach(const char *name);
      using <literal>InjectionPointDetach</literal>.
     </para>
 
+    <para>
+     An example can be found in
+     <filename>src/test/modules/injection_points</filename> in the PostgreSQL
+     source tree.
+    </para>
+
     <para>
      Enabling injections points requires
      <option>--enable-injection-points</option> with