Update event.md -a next proposal

[mpostol]

@BillWagner - I have just reviewed your last proposal and harmonized it with conceptual documentation. Hopefully, you will like it.

Summary

1. "a member that enables an object or class" - what about struct - it is not class and cannot be instantiated
2. "Clients" - in terms of language we don't have something like the client. I proposed to replace it with the user.
3. event handlers - I have added a definition harmonized with the Event handlers
4. "An instance of an event " - events are not able to be instantiated (they must be declared) in contrast to delegate value.

Rdgs,

Fixes #Issue_Number (if available)

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[BillWagner]

I like this @mpostol

I have a few grammar and word choice suggestions. Let me know what you think.

(Commenting rather than approving so it stays in my queue.)