filter: clarify comment (#13678)
filter: clarify comment (#13678)

* filter: clarify comment on FilterHeadersStatus::StopIteration and FilterDataStatus::StopIterationNoBuffer
* note that StopIteration refers only to header processing
  (data/body processing on current filter still moves forward)
* however, if a local reply is sent than StopIteration
  stops headers/data/trailers for all filters

Signed-off-by: Raul Gutierrez Segales <f25f6873bbbde69f1fe653b3e6bd40d543b8d0e0@pinterest.com>
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