feat: Adds clarification on registration process

Includes a note on user registration explaining that an
administrator must approve the registration before login
is possible. This aims to set proper user expectations
and expedite the registration process by encouraging the
provision of a registration reason.

src/synapse_registration/registration/templates/registration/complete_registration.html

@@ -21,6 +21,7 @@
             <div class="control">{{ form.registration_reason }}</div>
             <p class="help is-danger">{{ form.registration_reason.errors }}</p>
         </div>
+        <p class="help">Sending this form will create a Matrix account for you and reserve your username. However, you will not be able to log in until an administrator approves your registration. Please provide a reason for your registration to help us expedite the approval process.</p>
         <button type="submit" class="button is-link">Complete Registration</button>
     </form>
 {% endblock content %}