docs(hertz): fix outdated doc of hertz server config

[gaspardruan]

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docs: Documentation only changes

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The doc said WithReadBufferSize could limit the header size, but the code comments said not. And the code was modified(2025) after the documentation was submitted(2023). So I think it's good to update the WithReadBufferSize doc according the new code comments.

// WithReadBufferSize sets the size of each read buffer node in standard transport.
// NOTE: this cannot limit the header size.
func WithReadBufferSize(size int) config.Option {
	return config.Option{F: func(o *config.Options) {
		o.ReadBufferSize = size
	}}
}

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