Added description for HandlerInterceptor

I know that the posthandle is not intended to be executed when an error occurs in the handler . But the docs didn't show that. It says that called after HandlerAdapter actually invoked the handler, but it seems not enough. If this explanation is added, I think it will be a better docs for users.

thank you

Comment From: pivotal-cla

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Comment From: rstoyanchev

I've made a slightly different change. Thanks for the suggested update in any case!