api docs: improve wording of ext_proc ImmediateResponse.Details field (#19813)

I had been trying to figure out a good way to provide more internally-derived feedback from an *ext_proc* implementation to *envoy* (without needing to do things like set response headers or rely on a separate log file), and was led to the discovery that the `ImmediateResponse.Details` field actually populates `%RESPONSE_CODE_DETAILS%` for use in access logging.

Due to how much better this discovery made my life, I am hoping it will make other people's lives better as well.

Signed-off-by: Chet Nichols III <chet@apple.com>

Mirrored from https://github.com/envoyproxy/envoy @ e00aa66429181e3973cc69ca57c26da730cc47af

envoy/service/ext_proc/v3/external_processor.proto

@@ -298,7 +298,8 @@ message ImmediateResponse {
   GrpcStatus grpc_status = 4;
 
   // A string detailing why this local reply was sent, which may be included
-  // in log and debug output.
+  // in log and debug output (e.g. this populates the %RESPONSE_CODE_DETAILS%
+  // command operator field for use in access logging).
   string details = 5;
 }