📝 doc about the default headers

Signed-off-by: moznion <moznion@mail.moznion.net>

Author: moznion

README.md

@@ -225,6 +225,13 @@ If your default headers already include **all** required headers for the endpoin
 client.getUsersBulkJobid({path: {jobId: "123"}});
 ```
 
+NOTE:
+
+In this context, the "default HTTP headers" are different from the headers in `ClientOptions`.
+The headers in `ClientOptions` are always sent implicitly, regardless of the header parameters specified in endpoint methods.
+In contrast, the "default HTTP headers" mechanism is intended to reduce the need to repeatedly specify common header parameters
+in each endpoint method when their values are already known.
+
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