message: test: document new matchers
diff: @@ -58,6 +58,26 @@ Example:
  EXPECT_THAT(response->headers(), HeaderMapEqualRef(expected_headers));
  ```
  +### ProtoEq, ProtoEqIgnoringField, RepeatedProtoEq
  +
  +Tests equality of protobufs, with a variant that ignores the value (including
  +presence) of a single named field. Another variant can be used to compare two
  +instances of Protobuf::RepeatedPtrField element-by-element.
  +
  +Example:
  +
  +```cpp
  +envoy::api::v2::DeltaDiscoveryRequest expected_request;
  +// (not shown: set some fields of expected_request...)
  +EXPECT_CALL(async_stream_, sendMessage(ProtoEqIgnoringField(expected_request, "response_nonce"), false));
  +
  +response->mutable_resources()->Add();
  +response->mutable_resources()->Add();
  +response->mutable_resources()->Add();
  +// (not shown: do something to populate those empty added items...)
  +EXPECT_CALL(callbacks_, onConfigUpdate(RepeatedProtoEq(response->resources()), version));
  +```
  +
  ### IsSubsetOfHeaders and IsSupersetOfHeaders
  Tests that one `HeaderMap` argument contains every header in another
  