Document disposable email domain analytics parameter

[aduth]

🛠 Summary of changes

Updates user_registration_complete method signature to include the disposable_email_domain argument, which may be passed as a result of this code added in #9747:

identity-idp/app/controllers/sign_up/completions_controller.rb

Lines 104 to 106 in 43e0c4a

	if page_occurence.present? && DisposableEmailDomain.disposable?(email_domain)
	attributes[:disposable_email_domain] = email_domain
	end

Why?

• So that Analytics Events documentation accurately reflects the available properties
• To enable discoverability based on searches performed within the analytics_events.rb (e.g. this was motivated by my own pursuit to jog my memory about which event this was logged on)

📜 Testing Plan

After merged, observe the parameter is visible in Analytics Events documentation

Build should pass

[kevinsmaster5]

Looks like the spec needs to be revised to include added value.
I see, the spec will be done with #9747: