updates ServiceValidateController : JavaDoc that handles /proxyValidate

The `ServiceValidateController` Java class handles `/proxyValidate` as well as `/validate` and `/serviceValidate`.  This pull request updates `ServiceValidateController`'s JavaDoc to reflect.

`/proxyValidate` path --> `proxyValidateController` bean --> `ServiceValidateController` Java class.

that is, in `cas-servlet.xml`:

    <bean
      id="handlerMappingC"
      class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping"
      ... >
    <property name="mappings">
      <util:properties>
        ...
        <prop key="/proxyValidate">proxyValidateController</prop>

and

    <bean id="proxyValidateController" class="org.jasig.cas.web.ServiceValidateController"
        ... />